Most musicians, even many professionals, don’t seem to know how to properly execute the trill. In music composed before circa 1820, almost all trills are supposed to begin on the upper auxiliary note, NOT the main (written) note. It was Mozart’s pupil, Hummel, who spread the idea that trills on the piano should always begin with the main note; before that time, the only trills beginning on the main note were those immediately preceded by a short, quick main note, thus:
Otherwise, all trills were to begin on the upper note, with that note acting as an accented appoggiatura. Hummel should have known better. There were valid harmonic reasons for the old trill. Mozart would have been horrified. Unfortunately, Hummel was such an influential teacher in his day that he is still ruining our trills today.