That doesn't really explain it. Stuff that really needed to be dynamic would be. But we (I was at Yahoo 2003-2005) took privacy seriously and a lot of seeming quirks of login state etc. were well thought through and had good reasons. E.g. we were regularly reminded not to mingle data that identified browsers (that could "follow" an unlogged in user across sites) with data that identified users (as that would let us tie a userid to data the user explicitly had not knowingly shared with us).

And many particularly personal properties such as mail, or the billing system (my area), would take extra precautions about what information could be made available on other properties (e.g. what info from mail could be shown on the homepage) even if the user was logged in, to prevent leaking information that shouldn't leak. This would lead to extra logins that I'm sure seemed unnecessary, and logins where the user probably thought they were already logged in, but where non-personal information was keyed to the browser rather than their user id.

I'm sure there are bugs and unintentional quirks too - the system was crazy complex already in 2005, but I really hope they've stuck to their guns when it comes to how carefully they treated personal data back then.