@PPET pity Sense 2 doesn't show lap count during swimming so you could try to find out which lengths have not been counted and that may have given you idea why was that.
I use Sense and it's last watch that shows lengths on screen. I can pinpoint a few reasons of undercounting laps:
- not strong push-off. New length starts when there is a sudden change in direction detected. Smooth and slow turn around may result continuing the same length by not detecting length.
- (this happened a few times although it's rare) new length from rest doesn't register and watch starts counting when turning around after first 25m.
- swimming with hand paddles or pull buoy may result with slower, less controlled turn and confuse the watch. That was happening a lot to me when using hand paddles, without doing flip turn it was too hard to turn around and easy to confuse the watch.
- drills when not using hands or using hands strange way (not only swimming with kickboard but some drills like side-kick drill which use arms less usual way).
From numbers I see you pool length is set correctly. All I can say is that Fitbit isn't really great as a swim tracker. Swim tracking is there but it's kind of rushed implementation without giving a thought what swimmers actually need. Not much can be done about it.
