Ask HN: Why does Gmail sign in POST to accounts.youtube.com?

I have noticed for a while that when I login to Gmail via the multi-account selection screen, the sign in posts to accounts.youtube.com, while signing into youtube results in a post to accounts.google.com.

Do any googlers know why this decision has been made?

It makes me wonder GOOG makes any interesting infrastructure/cost decisions based on how it will help their financials. E.g. have YouTube take the costs of providing certain services to the rest of Google, etc.