Play less Engineer. Seriously. It's mindboggling. Go play Soldier or something fun!
I do enjoy engineer (not as much as the stats might suggest), but I've banned myself from playing it for a while because it really is getting a bit silly looking at those stats, yes...
Simple and obvious explanation is that when you're on that class, you tend to stay on it, because your team is relying on your sentry, teleporters etc.
As for soldier being fun, well, he's easily one of my most effective classes, and it is fun, but I don't enjoy it as much as I should because a) it's so easy that it feels wrong and b) too slow (movement AND rocket speed, remember I'm a proper Quake fan).
If you really care the stats actually lie because they got partially reset at one point... all my medic, soldier and pyro time got reset to 0 (Pyro and Soldier were at 20-30 hours at the time).
p.s. I'm not alone:
http://www.steampowered.com/status/tf2/tf2_stats.php