Just my 2 cents and speculation, but is it possible it has something to do with keyboard layout? Are you sure you typed your password correctly if it was a different keyboard layout?
I have several user accounts aswell with passwords, which all are configured with another keyboard layout. However, if i restart the phone, the keyboard layout to login is always US qwerty. If i type my regular pattern fast, my passwords are also 'wrong', because some keys are different on us qwerty and my keyboard. Could this be the case here?