When K2 starts a match, he has a certain protection level, referred to as max protection. Let's say this is 50k. If he gains 1% max protection, he is now at 50,500 max protection. Another 1% gain puts him at 51,005 max protection. This does not mean he currently has that. But when coupled with someone that can heal protection (like Jyn zeta lead, or HYoda), you can now heal him up to that new value.
STH, on the other hand, gets hit, and then gains 5% protection regeneration after the hit. His total allowable protection stays the same, though.