I too have experienced this. The reason given was that conductors doing a visual inspection of season tickets generally only look at the expiry date. So if you buy a ticket this Wednesday to start next Monday, you could potentially get away with using it this Thursday and Friday. Now given that most of us wouldn't think of doing anything dodgy like that, booking clerks used to bend this "rule" frequently. I am told, however, that some of the new generation of ticket machines enforce the rules rigidly, with no over-ride available.
I can understand that ... and no doubt you're right. Yet the question was
renewal, so if you're looking to cheat the system what are you going to do - buy the new ticket then give the old one to a fellow traveller so he can use it for the final day or two? I guess giving staff machines that cannot sell the seasons early avoids any arguments ....