1x3 gives an odd number whereas 2x1 and 2 x 5 given even numbers
13 is the only prime?
13 is the only one of the three numbers that appears on a dartboard
It's the only one that's 4 ,ore and 3 less than a square number.
Far too many possible reasons.
Was this the full list/question? Was there anything there to connect it to roulette?