In reply to Scott Anderson:
Had a similar problem with a Fenix and my iPhone.
So, calibrated my watch exactly to a contour line, walked up a couple of hundred metres then back down to the same point and the watch was fine but the phone was out by 12 metres.
Most watches I've had have been fine using them for altitude and I'd never trust the phone as I was told by a Suunto chap the watches were designed for this use whereas the phones are not.