Hi, I've had some really good and really bad reads from swims in a pool - the same pool in fact. Any tips on more accurate logging assistance?

For some context, I am a very consistent pace swimmer and swim 60 lengths in 30 minutes, I often manually count. Last time I did fitbit told me I'd swam for just 21 minutes and completed 37 lengths. I obviously then can't change that except to delete it and re-add manually, right?