Should be just the swelling.
iPhones put out a lot less Oomph than ACRs, in general, so that could explain it. 
I am more interested in how hard is it to get a read (i.e. how many times you need to pass through, or if you need to try and angle the hand in a special way).
I’m starting to think that either I got an underperforming new antenna, or, most likely, an exceedingly overperforming old antenna.
Anyway, here’s a long Thread I’m putting up about it, so we can stop derailing this thread! 