I'm not quite sure of the connection between my comment and Eric Clapton/Nathan East, so I'll have to assume that Eric and Nathan don't have their instruments tuned the same...? Given that Nathan usually plays a five-string, he has the option of transposing to hit anything below 440 E; I play a 4-banger.
Anyway, my point is that there are times (as in the case of my current original band) that I have to drop-tune a half step to match their tuning (actually, 1.5 steps for the E string; D-tuned; pun), then retune to 440 for the hired-gun gigs I do (or to woodshed most cover songs).