Hi WLTU,
Good point on the 60 then 56 KIAS and I fully expect that, assuming the manufacturers actually fully test their aircraft for the numbers they put in their Handbooks, that there is variation in the techniques used to obtain the numbers and in the execution of those techniques; However,
the climb rates in my spread sheet are average climb rates for both Vx & Vy. If the manufacturers statements for ground roll and total distance over a 50' obstacle are fairly accurate and correct (and the 50' is accurate and correct), then the angle of climb for Vx should be fairly accurate.
Changing (for instance) the '64 150 IAS from 64 to 60 changes the average climb rate from 433 fpm to 406 fpm, which for the 60 to 56 KIAS you quoted would band the uncertainty in the spread sheets average climb rate numbers - IOW a possible error in average climb rate of +/-14 fpm or +/-3.3% in the '64 150 case.

This point does bring up 2 other problems:
1) If one were to fly the Vx obstacle clearance numbers in POHs EXACTLY (somehow) would the wheels be rolling over the top of that 50' obstacle? - or is there some margin of safety built into the numbers the POH is silent about. e.g. is it actually a 55' or 60' obstacle the POH numbers are good for?
2) Why is there no specification of ground roll distance for a lift off at Vy in any of the POHs I looked at and not required in the GAMA specification? It seems that this distance would be closest to the distance of a normal TO roll, and therefore an important number to include.