I, too, have noticed the Garmin-connect reported RHR floats higher than what I directly measure and the lowest BPM that I see in my 4-hour window on the watch, too. It seems to do a great job with tracking my heart rate during exercise, shadowing my Garmin heart rate strap very closely on the trainer and the treadmill. For me, that’s the most important feature, along with gps distance for my outdoor runs which also works very well.
I keep the watch fairly close to my wrist (I’d say within ~1"), and reasonably tight with minimal slack but no discomfort. I just tried scooting it up about 3/4", and the heart rates being reported were in the same range.
I guess I won’t worry about the occasional low heart rate readings. I’ve seen 31-33bmp a few times, but not frequently.