Climb rate and how specifications are cited

[YankeeClipper]

I see that the later, N/A Skymasters (the II's) are rated at 940 fpm climb, vs much more on the F (or any earlier one). The empty weight is also considerably more, but I thought that specs were nearly always cited per full gross, which is the same for all of the above. Are they not per gross, or is there some reason that the later models are slow climbers, in spite of having the same gross numbers as the F?

Thanks,

Gray.