I don't know what a power meter is, but the last time I rode 2.5 hours I covered 58 km. According to my low-budget cycling computer which judges calories burned based on age, velocity, time, and weight that trip burned ~3300 kcal (it also suggests 0.85 lbs of fat burned, but obviously not every calorie burned came from fat). The calculation is obviously clueless as to wind-resistance and up/down hills, which will skew the results.
I don't have much else to go on, but I can say that on average I've been dropping almost as much weight as it says, so I believe it must be at least in the vicinity of accurate. On the other hand, your 2.5 hour ride apparently burned less than half as many calories and people are saying that your numbers are inflated.
Summary: colour me confused -- I never expected my computer to be terribly accurate, but based on my weight lost it certainly seems to be close to the mark; meanwhile your numbers are in a totally different ballpark from me.
I'll have to check out this "power meter" gizmo....