Success rates

I dunno if I should put this in, but despite having a difficulty of "Master", so far when I played Barry's given songs, out of twenty-five tries I've got fourteen "very good" results, eight "successful" results, two "bad" and one "very bad".

What's strange is that my Playing Instrument rank was F. (And for the record, Composing is Novice, I don't even have Musical Knowledge, and I was using a Siren's Flute.)

Was this the score's property, to make playing the song easier, or did Playing Instrument's formulas change when I wasn't looking? Or maybe the comparatively high number of "very good" results were just a fluke?