This. I don't use gmail, but I use youtube. And when you've logged into youtube, you've logged into your "Google Account". So you can't have youtube open in one tab, and not have your history from other (non-incognito) tabs logged (modulus some tweaking of various preferences that aren't obvious or intuitive).

There's no reason why they couldn't to the same thing Mozilla does: a) have a pretty clear account thing for "sync", b) have a pretty clear page for opt/in out on what to sync, c) Have working self-host sync solution, d) have an open source sync solution so you can easily see what's going on, and how things are encoded/stored.