The only thing I can think of that might be causing this (having just registered an account as a test) is if you chose to skip the verification code when registering, in which case Google may have sent a text to your phone with an account activation link or something of that nature. Many websites allow you to use an unactivated account on the machine it was registered on, but then don't let you log out and back in again until it has been activated. They also don't let you log in with another device. If that doesn't help, try logging out on your PC before trying to log in on your tablet. I don't think this is likely to be the problem though, as I have had several machines logged on to the same account simultaneously.