Bob's right. The normal cross harp to use for a tune in C is F, but I just tried Amazing Grace in C on both a low F and a G harp and had no trouble finding the notes for the straight melody on either harp.
I just tried it on the C harp and in order to get the notes without some severe bending I had to play it in the high octave. You'd have to do the same if you use a regular F harp, so perhaps Vixen's dad didn't have a low F and wanted to play in the lower octave. It sounds pretty squeaky up there in the high octave.
I guess it just depends on what embellishments you want to use as well.