I notice mine fluctuate during the year too. That’s the great thing about Aetna tachs, you always know for sure. (I think you run those too)
I have 2 sets of sea ray analog tachs in storage to prove the hours if need be, otherwise, they are worthless.
oh yea, the aetnas might be the best upgrade available. I put them in the weekend we bought the boat. EGT and boost are a close second in my opinion