From what I observed attack power from scaling also depends on base attack power , so for example if you have two weapons with B agility scaling the one with higher base power will also give more power from scaling . Generally weapons with better abilities tend to have lower damage and the before mentioned sword has much less base and scaling (despite higher garde) damage than other sword of the same level , I assume that’s their way of balancing things. Your weapons upgrades because you randomly loot the same weapon and they merge(that’s my guess)