What is the difference between "site:current-user:field-foo" and "account:field-foo" in the Rules "Add a new condition"?
As I mentioned in the comments the variables you have available to you in a rule depend on the context it's in.
[site:current-user] is a global token available to any rule and will always return an object relating to the currently logged in user.
[account] token is there because you're responding to the "After saving a new user account" event; that event passes an object relating to the user object that has just been saved (for obvious reasons).
I guess the main distinction is that the former will always give the currently logged in user, while the latter could be a different user object altogether (if, for example, an administrator had created the account).
In the "After saving a new user account" event, [account:field-foo] is referring to the account being saved, while [site:current-user:field-foo] is referring to the currently logged-in user, which possibly is the user who created the new account, or caused the account to be automatically created.
In the case of an administrator creating an account for another user, [site:current-user:field-foo] would be for the administrator account.