First, that check is just a guide and is not guaranteed to be accurate. Only a BT line test can prove if ADSL is possible and the only way for that to happen is to apply for ADSL to be activated on your line. That check said I could not have ADSL, been connected for over 2 years now!
2 lines can easily be different. While they may both terminate at your house, they may take totally different routes to get to the exchange, and one route may take a longer path and thus it is effectively further away from the exchange. One of the key factors in determining if you can have ADSL is the distance your line is from the exchange so that is probably the reason why one passes this check and the other does not.