You have to factor in the fact that dps classes are far more efficient with their potency per second, being able to turn their increased attack power into even further increased dps better than tanks can turn attack power into dps. This adds a slight but present multiplier to the speed at which the differences will increase. Again, it's slow burning, but it is a problem.

Additionally as has been stated, having such low scaling can make i320 choices best in slot over i340, maybe even up to i380+ over more efficient secondary stats alone. It's not hard to beat out 2, 4, or even 6 or maybe 8 str depending on what secondaries you are trading.