Unrealistic Swim Records

The most important thing with swimming is the turn around and push off as that is how it knows the number of laps and hence the distance.

I am assuming it shows many less laps than you actually swam , hence the distances are less.

See this support note

support.garmin.com/.../

The link is correct, but the conclusion is the opposite IMHO: it thinks you swam much more than actually, thus it divides a longer distance by the same time. I saw it even when I swam * and even more when my kids swim. 

*) one thing I was recommended and seems to work well is to disable automatic pause detection. If I really pause I hit the lap button when I stop, that starts the rest period, then I hit again and it starts the next swim period.

Other things: don't change stroke type in the middle of a lap

Don't stop in the middle of the pool.

Try to kick yourself from the wall every time you start a pool.

I also set up the main screen so I see the total distance (or number of laps, depending what you're counting in your head) Sometimes I have a look during the swim to confirm it counts good enough.

Right my brain was in reverse.