I, too, use the PoolMate, and the results can be interesting. I've found that if I don't push and hold the button firmly to finish that the PoolMate will add a subworkout of a few seconds and zero strokes. Sometimes it adds a length, too. For the total workout it will average the stroke count between the real workout and the zero workout, effectively cutting the stroke count in half. You have to check the log to be sure there isn't a three second, zero stroke subworkout mixed in.
On a swim in the 50m lane last week, I lost a certain count of laps when a slow-swimming woman with a snorkel suddenly appeared in front of me. From my time I thought I had swum a slowish 1600, but the PoolMate told me I swam 1650 — which was impossible since I started and finished on the same wall.
How to count? You just have to tune out the other concerns, pick a uniform time to count, such as when the fingers break the surface, and say each number inside your head.