When a song is mastered, you don't necessarily put the whole album on a single tape, you do each song separately and then aggregate, thus the chance of accidentally speeding up a few songs. Remember, this was WAY before Pro Tools and hard drives. And if you don't know about how fickle tape machines are (and the amazing things that happen when you mess with speed, touch the reels, etc), head over to Gearslutz and search. The old-school sound engineers have tons of tricks to make cool sounds using reel-to-reel tapes. IIRC, Megadeth tuned down a quarter step as well on some of their earlier albums.