Like the other commenter says, it's probably a matter of how your arm is (both position and not moving it at all) during the testing, those automatic machines are super fickle about it compared to the manual "pumps" ones.

Your doctor should see it though, unless they don't stay next to you and thus can't (but I know mine use it more like a first control test, and would always do a manual test before prescribing anything).

YMMV, I'm in France.

PS : with that said, this here is even more different since it's in a watch, and I would not trust it for anything other than "hey, maybe go check at a doctor"