There are at least two recordings on Youtube of Bach's Brandenburg Concerto No. 5 in D major which seem to be played a half-step lower, in the key of C sharp. I assume this is a artifact of the recording process or uploading to Youtube but wonder if there isn't some tradition of which I am ignorant in which older pieces are thought to have been played in a register other than that determined by current standards (with their very precise association of wavelength and musical notes)?
The first example at 18:43-44 gives the sustained D at the end of the piece. The second example gives the same sustained note at 19:36-38, but according to the various online tuners (here is one) the first example is D and the second is C-sharp.
Here is another example in which the piece seems to be played in C sharp major instead of D. The final sustained note at 21:23-24 can again be checked with any online tuner or compared to the first example above.
Obviously this doesn't affect the quality of the recordings or performances, but that it occurs more than once makes me wonder what's behind it.
Any insights appreciated.