The unwritten rule is that when an administration is reelected, with the commander-in-chief intact, the numerology remains the same. That's why we only have 44 presidents up until this point, and not 56. That's why FDR is our 32nd president, but not also our 33rd, 34th, and 35th.
Interestingly enough, even though Obama is our 44th president, 43 PEOPLE have been president. That's because big fat stupid Grover Cleveland, elected non-consecutively, is our 22nd AND 24th president. Jerk.