The fenix 2, Forerunner 910XT, and Garmin Swim uses arm movements and preconfigured data (such as pool size) to provide swim metrics.
Discrepancies in distance may be caused by multiple sources:
To decrease the amount of discrepancies:
As swimming technique improves with the use of the above instructions, distance accuracy will improve.
1 - If drill log is not available; use rest intervals for drills that affect arm movement
2 - If lengths are being missed (under-counted) in a small pool, consider using a standard-sized pool as the watch may miss lengths for strong swimmers who only take a few strokes to cross small pools