And it's not gerrymandering, either. The recent round reduced the number of single-digit presidential districts, but only to 74 (based on 2008 results) -- far more than the 33 House districts that are being decided by single digits this year.
Even many Senate races that were supposed to be "toss-ups" -- Mitch McConnell, Pat Roberts and Arkansas, for instance -- wound up being double-digit races.
A similar thing happened in the House, where Republicans largely either won "toss-up" races convincingly or lost them. As of now, only nine GOP victories occurred by single digits, with 11 races still outstanding. Democrats, meanwhile, have won 15 of the single-digit races that have been called by the Associated Press.
Here's a look at just how close -- and not close -- all the House and Senate races were (with the important caveat that nine undetermined House races and three undetermined Senate races are not represented here yet). Notice the dip once you get into single digits:
What's most interesting about the above is that the Senate chart looks a lot like it should in a wave election -- with Republicans pulling out a lot of close races and Democrats winning only one so far. The House chart looks a little weirder, with most close races going to the party that lost ground, the Democrats. But again, this is because the GOP won so many "toss-up" districts by wide margins, and because so many unexpected races wound up pretty close for Democrats.
David Wasserman summed it up well here:
As Wasserman notes, if you look at the races Democrats won by single digits, a lot of them were in what was supposed to be safe territory -- California, Hawaii, an un-targeted district in southern Minnesota, a heavily Hispanic district in Arizona and even Rep. Emanuel Cleaver's (D-Mo.) district, which went for President Obama by 20 points in 2012.
The result is surprisingly few members entering the 114th Congress thanks to a close shave. But for plenty of them, that won't always be the case.