The three most common reasons are:
1) You claimed differing days of "personal use" of the property for those years of change.
2) You reported the property rented for less then 365 days of the year.
3) You took the property "out of service" for differing periods of time during those years.
My guess in your case would be #2. I see that most often. Periods of vacancy between renters counts as "days rented" provided you did not live in or utilize the property for personal use for one single day during that vacancy.