I don't understand Mastery

Yes. I should have corrected my previous sentence. STR does not add less than DEX. It nearly always adds more (still assuming, as before, we're talking swords). It's just that it tends to spread your damage range apart, while DEX does not.

Going to sleep for real now.

This is correct. Going by the formula, one point in STR adds a lot of maximum damage and a bit of minimum damage, but that bit is still larger than what one point in DEX gives.