I guess. That's not immediately clear to me. However, browsing around on Google Books suggests to me that it is the corpus which changed, not the algorithms.

> surely the newer edition is going to be preferred by most readers.

Why? Where different editions exist, the reader will want to know which one they're getting, but they're unlikely to systematically prefer newer editions.

But also, Google Books isn't aimed at "readers". You're not supposed to read books through it. It's aimed at searchers. Searchers are even less likely to prefer newer editions.