As the medical expires after one year and in rare circumstamces 3 months extension may be given if the file is close to completion.
However, in my case security background checks took about 14 months, so I was asked to redo the medical. Security checks may be delayed due to multiple reasons and in my case yes there were a few factors which may have delayed them. I have stayed both (worked and studied) in different countries including USA, UK for a few years. It could be just due to your name, address and profession. For regulated professions they do much extensive security checks as I have applied under Social Worker category which may have been another reason of this delay. It just varies from case to case.
Good luck and as you still have time left in expiry of your medical, hope that you do not have to go through them again.