Here is an example to help with the concept of linearity and stat caps:



That is a graph of Lancer auto-attack and the influence of STR. Somewhere around 285-290 you can clearly see a change in the slope. All the damage bonus from 290+ STR is from the ~2:3 ATK:STR ratio. The bonus from STR itself caps around 290 and that is why you see a change in slope.

A little off topic, but hope it helps