It happens very often in early music that the keys are notated with one less flat. Hence, we see a score in Bb major with just a flat instead of two, a D minor piece with no flat at all, and so on.

Since we tend to conserve the original key of the early music we transcribe, you may find such oddities in our editions often; hence, if you want to be sure about the key of a piece, do not count its accidentals; instead, look at the very last note of the lower part.