Harmonicas are often tuned higher than A=440, the rationale often given is that they give a "brighter" sound. They do however have a tendency to play flat with more air pressure (notwithstanding deliberate bending of notes), so harder playing will tend to bring them back down to concert pitch (rather than take it below).

It's also quite common for diatonic harmonicas to be tuned to just intonation (or a compromised temperament) rather than equal temperament, because chords sound smoother that way. This means that some notes will be up to 10 cents flat of their equal tempered equivalents. Starting higher than concert pitch means even the flatter ones aren't so far south when playing with equal-tempered instruments.