You'd be amazed how easily guitar players at this level forget what tuning they're in, because they never really have to tune their own guitars. They commit the actual playing to muscle memory, but unless they have perfect pitch they probably have no idea if they're in E, Eb, D, or whatever. That's why when a guitar tech messes up and hands someone a guitar in the wrong tuning, it's usually a trainwreck (see: Points of Authority from KFMA Day 2014).
True story that I've never shared publicly, but screw it, enough time's probably passed by now: When LP did Hybrid Theory in full at Download 2014, the crew contacted me to find out what tuning Forgotten was in. It had been so long since they played it that none of the guys who worked on guitars during the Hybrid Theory era were working for the band anymore, so they didn't have a clue if it was drop-tuned or not.