I've noticed that sometimes a different recording of the same piece will sound like it's being played in a different key. For example, I listened to Concerto No. 6 in B-Flat Major, BWV 1051: III. Allegro in a recording by Richard Egaar and then one recorded English Baroque Soloists. When I compared the two, the Egaar version sounded to me like it was performed a semi-tone higher than the EBS recording. If the piece is in B-Flat Major, shouldn't both performances sound like they are in the same key?