The 1660 gives approx 22MH/s. Some cards perform bit lower than the expected MH/s irrespective of whatever OC settings. We have to remember that same brand GPU of same model with same memory chip and batch number performs differently.
There are gtx1660 graphic cards that do 27MH using Trex absolute core clock settings but all the same it's all about silicon lottery but the minimum I've seen are 24MH to 25MH if yours is getting 22MH it's the silicon or you don't get the proper clocking