According to musicologists we have been listening to Robert Johnson’s recordings all these decades about 20% faster than the way they were originally recorded..!!
The reasons are not exactly known but speculations include accidentally sped up when first committed to the 78 rpm discs and intentionally sped up to make them sound more exciting. Slowed down versions are now starting to appear on the internet.
Recordings by other artists may have had the tempo changed during the mastering process including; The Doors, Miles Davis, The Rolling Stones and Bob Dylan to name a few…. This may explain the reason for having to slightly alter your guitar’s tuning from A440 to jam along with your favorite recording..!! Nice one Alan. Thanks..!!!