I've seen this one before. A friend had this restiction.
Basically, his situation was such that a prior medical condition caused
the FAA to decide that he needed to renew his medical every year. So
if he gets a 2nd class medical, it doesn't turn into a 3rd after the
year. If he gets a 3rd class medical it expires at the end of 1 year
and he has to redo the medical.