I don't know if Kubota is overly conservative with PTO ratings, but, I find it a bit odd that when you have several models that are essentially the same tractor, but with different HP engines, that the higher HP models seem to loose more PTO power to the hydrostatic. I can see the drawbar power being a % of engine power, and thus the model with the larger engine will lose more HP getting that power to the wheels, but the PTO doesn't use the HST.
Case in point:
B7610 - engine=24HP, PTO=18HP
B7510 - engine=21HP, PTO = 16HP.
The B7610 has 3 more engine HP, but only 2 more PTO HP. The B2920, B2620, and B2310 have similar differnces (there is like 3 engine HP between each of these three models, but 2 PTO HP).
I would think that because the tractors weight about the same, the the power required to move the tractor with the HST would be about the same, regardless of available engine HP, and that the full difference in engine HP would be available at the PTO. Also, would more than this PTO HP spec be available for stationary use (such a
chipper or generator), since the HST isn't being used?