There is, of course, no such thing as a lock in sports betting. The naive belief that there is has allowed dozens of people to make lots of money selling systems to suckers. There is one thing that has been pretty close to a lock this year in the NFL, though - the double-digit underdog. Teams have been favored by 10 or more points 13 times in the first eight weeks of this season. The underdog has covered in 12 of those games. Only the Giants, favored by 10 against the Niners in Week 7, were able to buck the trend, and they only managed it by two points. Not only that, but four of those 13 underdogs have won their games outright - a rate high enough to show a profit on the money line given the huge odds they would have paid out at.

Obviously, every time you see a trend this one-sided you have to assume that it is a fluke. If it weren't then it would be too good to be true. To give a quick test of the relevance of this little anomaly, let's look back at the last two seasons. For the sake of consistency, we'll just look at the first eight weeks of each season:

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2007 - Double digits dogs were just 6-8 ATS in the first eight weeks of the season. That means it was narrowly profitable to bet the favorites, but not to an extent that made it worthwhile. There was just one outright underdog winner in the 14 games, so betting the dogs on the money line would have been an unqualified disaster.

2006 - In a strange coincidence, double-digit dogs were again 6-8 ATS in the first eight weeks, and again there was just one outright winner.

That means, then, that this year is, at least in recent times, a bizarre anomaly. Betting on the underdogs in games with a double-digit spread has not been close to profitable the last two years, and this year it's been a license to print money. The number of games with the giant spreads we have seen is consistent, so why are the results so different? It's hard to tell for sure, but here are four reasons that are certainly contributing to it.

Tom Brady's knee - Last year, the Patriots were favored by double digits in four of their first eight games. They covered each time, and easily in all but one case. This year, their stature as a significant public team has seen them favored by double digits twice. Matt Cassel has made it very clear that he is no Tom Brady, though - the team hasn't covered either spread, and lost to Miami outright as 12.5-point favorites. The Patriots single-handedly account for a large part of the swing between last year and this year.

Inconsistent performance among top teams - In the first few weeks of the season especially, the public is guided in their opinions of teams more by what the experts said in their season previews than by what the teams are doing on the field. There are a significant number of high profile teams this year that are doing far worse at this point than they were supposed to be. Indianapolis, San Diego, Seattle, Minnesota, and Jacksonville were all seen as teams with a good chance of winning their division and going deep into the playoffs. None of those teams are even at .500 at this point. Add in a Dallas team that has forgotten how to play recently and the Patriots minus Brady and it's no wonder that the underdogs keep having their day. The public won't give up their perceptions of these teams regardless of how much evidence there is that they should. The teams I mentioned are 0-6 ATS as double-digit favorites. Things are so bleak in Seattle that they actually were double-digit underdogs once this year. They obviously covered. It's hard to know for sure, but my strong sense is that the lack of strong performance among the team's with high expectations is much more pronounced this year than in recent years.

After you finish this NFL article check out our NFL Season Win Totals page. Doc's How parlay wagers work resource is a must read for NFL wagering. Doc's Sports NFL preseason betting tips page is and excellent NFL resource as well. For more NFL articles and free picks visit our homepage and view the "Doc's Daily Medicine" section.

The worst teams just aren't that bad - It might seem from our current perspective that the Bengals, Lions, Rams and Chiefs are historically bad. They may very well be. They just aren't as bad as people think. Those four teams may have only combined to win three games in 29 tries, but they are a perfect 7-0 ATS as double-digit underdogs. That's especially significant considering that the four teams are only a combined 10-19 ATS overall. The public generally overestimates the degree to which a bad team stinks, and this year is proof of that in the extreme.

Offensive parity - Last year there were 12 teams that average 20 or fewer points per game on offense. This year there are just seven so far. Ten points is a lot to win by, and when fewer teams are struggling offensively it is a big disadvantage to overcome.

So, What does it all mean? - For bettors, not much unfortunately. We know that this year is out of whack, and that sooner or later it is going to correct itself, and the favorites will win more than their share. The biggest mistake any bettor can make, though, is assuming that that correction is going to happen now. Even if a coin has come up heads 15 times in a row, the likelihood of tails the next time is just 50 percent. In the same way, assuming that this stat is going to correct itself now is a good way to go broke. It might be more balanced by the end of the season, or it might take a few years for it to return to average.