Not sure it's limited to the Fossil, seeing the same on the old LG G Watch R, I think it starts a scan when it detects movement after a rest on the gyroscope input, rather than when it's on the wrist and unlocked.

Misreads seem common on any device, unless the strap is very snug and... well, you never take it off to charge.

It's worth bearing in mind the data it generates is only useful over time with apps like Cardiogram, so I've found it's best to set quite a relaxed interval for reads (every 10m) as it gives you a decent average over the hour, but I'm still having to try and delete weird misreads.

Also, your heart rate does change much more frequently than you would expect, a particularly stressful TV episode could get you up to 130 for a while.