I think that the growth curve symbolizes just that -- growth. The amount by which the stats increase with each level up, not their hard-and-fast values for each level. If Chess is level 1 with 5 STR and will gain 10 STR when he reaches level 2, and I give him some back-alley medication to increase his STR by 5, he should have 20 STR at level 2 -- 5 from level 1, 5 from the back-alley meds, and 10 from the level up.
Then again, I'm not sure how the formula works, but it only seems reasonable to have a growth curve symbolize growth and not the stats themselves.