I found this tier 3 Sai pair as a +2 (5-14 damage). But since T3 Sai +0 do 3-10 damage, I expected the +2 to do 5-18 (since normally weapon upgrades add 1-(T+1).
The 1/2-normal upgrade pattern continued over time too, so now at +4 it does only 7-18 (not 7-26).
…Is this weapon “penalized” to make up for the first finding it as better-than-normal +2?
((I’m trying to resist the urge to go digging in source code & take away the fun/mystery))
I think it’s penalized for its fast attack speed. It gets two hits per round instead of the normal one, so it makes a kind of sense for it to only get 1/2 the normal attack upgrades.