Changes in the sound of the recording would most likely be from slight variations in the turntable speed. Even if he adjusted the pitch control for each album before playing it, it would be difficult, if not impossible to have each one turning at exactly the same speed without some slight differences occurring during playback.
He seems like a kook with too much time on his hands...
Rutherford Chang – We Buy White Albums : Dust & Grooves ~ Vinyl. Music. Culture