What the guys above said. ALWAYS end your reserve month with a day off and hope not to get rerouted into the day off.
Think of it this way. When you look at your reserve month, you are on call everyday there is NOT something next to a date like XX or ** or training etc. On a regular month you are off unless there is a trip next to it. However, going from reserve to regular month, scheduling looks at the regular month with nothing next to them as available days to finish up any reserve month carryouts. Also, on a regular month, this is how they are able to inverse assign.
Not an expert so anyone else please weigh in, but that's the way it was explained to me when I complained about the same thing.
Ferd