Hi @Pu.Chen, and welcome to the forums!
I think @tqFan pretty much covered it, but this
makes me think that maybe you’re comparing the grimtools link to your in-game character sheet(?) since even with all buffs turned off, the grimtools link reads 83% with 2% overcap (where the 83% instead of 80% as the cap is coming from the Shattered Realm Ward amulet). If this is the case and you have basically the same gear, then the remaining difference in pierce resistance values likely comes from slightly low pierce resistance rolls on your gear. (Grimtools displays resistance values on the character sheet based on average gear rolls, so some variation is expected when comparing to in-game values.)
(This may or may not be unnecessary info for you, but I figured I’d chime in just in case.)