Many thanks, Machelett.
Yes, all in all this is not so critical a factor that it would spoil my swim at all. I can always count myself from time to time, as you say, I've no problem with that.
But for an argument sake, I would point out that:
1. Garmin Swim promotional video suggests it counts every stoke (each hand movement) - watch the video at http://sites.garmin.com/en-US/swim/
- look at 0:54 sec. of the movie and text displayed.
2. I thought this piece of kit was so clever that it would double the count itself...
3. I would have thought, the watch would actually add odd strokes when involuntary movements, not reduce them...
4. Doubling 9no. strokes up gives me 18 strokes, while I was doing 14... [edit: for a lap (2x lengths) I would have 28 strokes.., again not 18]
5. No one complained before, so I thought it had to be my problem. Does your watch count correctly? Does any other watch (forerunner?) count correctly?
As I said, it should not be a huge problem to me, though I will still try to figure out what's going on during my next swims.