It's actually worse than that - the account is indicated in the URL, but it's expressed as an offset into a list of accounts in the order you signed into them!
So a ".../mail/u/0/" means the first Google account you're logged into, ".../mail/u/1/" means the second, etc. But if you did it in a different order on a different machine (or even browser), it'll redirect to a completely different one.
I would love to have been a fly on the wall in the meeting where it was decided that was a good idea.
Somehow they made it impossible to specify or indicate the account in the URL to squash any uncertainty, you just have to hope the cookies are right.