You are expecting too much imo, post shiryu fight Zoro mid-diff Kizaru max, while post Mihawk fight Zoro low diff, that is the thing. Current Zoro only version Kizaru might puss to very high diff.
Admirals become irrelevant power-wise after this arc and this also means Oda won't hold Zoro back anymore against them since Luffy already fought Kizaru an admiral so there is no point in hyping them anymore.