I don't know if I can trust recount. Maybe I have it configured incorrectly, but after talking with some people I ran with in WotLK, I did 2k more dps than my own recount showed for some reason, as a petless class to boot.
So I dunno what the deal with it is. I generally just use it to see if I'm really slacking or whatever, not as a personal measurement tool.
Recount likes to share data between other recount users I believe, which makes for more average results. If you were getting completely outlying numbers, then you were probably looking at the wrong data, or a bug happened on your end. From my experience, Recount is very accurate.
And DPS will never be completely predictable. Factor in parries(doingitwrong)/dodges/blocks/misses, crit chance, the fact that damage is done in a range and not a set number, all of the other factors such as mechanics in the individual encounters, etc, you can get huge ranges from the same person in just one dungeon.
A good player in crap gear can do amazing DPS. A bad player in awesome gear could do crap DPS. If you blow your load on the boss 2 seconds after you pull, you can do some ridiculous amount of DPS, and still do almost no damage because you died right away. Not sure if they fixed it or not, but I know before once you died and all of your dots are off the table, your DPS stays unmodified for the rest of the fight. So, like I said before, go nuts on the boss and die immediately could yield you some insanely high numbers that nobody else can catch up on.
Besides, what matters anyway is damage done. I don't care if you can do 15k DPS but still can't do more than the tank, you're useless.