1/2 expected damage upgrade, from finding +2?

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))