What you have not stated is how the rate of climb compares either with the graphs, or with varying IAS. Surely, the point is that the R of C is a function of the TAS and that the IAS slected will reflect this. Hence the fact that the IAS will should normally decrease by about 1 kt per 1000 ft in the climb (about 1% per 600 ft actually, but given the average Vys around and the need to be practical, 1 kt per 1000 ft is how it's written).
You state the relationship with GPS g/s, had you calculated the TAS under the conditions? Was there really no wind affect?
In general, it is true to say a manufacturere will want to give the best possible figures for an a/c, and the regulator will make quite sure they are not exagerating them before certifying the FLM.