These stats aren't correct. I'm quite sure they haven't been counting since I started using the SDK. I calculated making/testing/playtesting hoodoo took 2000 hours of my time.
Even if I used the SDK for an average of just 43 hours per two weeks for one single year I'd have used the same as what it tells me I've used for all time. And I'm quite sure that it has been >50hrs per two weeks pretty much constantly for the last two and a half years.
If I diminish that average to an estimate of 15 hours a week over the last two and a half years, that gives me a total of just under 2000 hours in the SDK. I believe I've spent far far more than an average of 15 hours per week.
What does steam tell me? 1120.8 hours.
Do I believe it? Not for a second.
EDIT:
Further proof these stats are incorrect: It only lists 12.9 hours total for my L4D play. I shouldn't have to say any more really, these stats are just ridiculously inaccurate.
(TF2 is 1910.4hrs, HL2 is 337 hrs.)