Looting 3 does not work with wither skeletons in 1.16.1

I have a looting 3 sword which, when killing wither skeletons, should allow them to drop wither skulls at a rate of 5.5 percent vs 2.5 without looting. This means with a looting 3 sword, I should get at least one wither skull per 18.18 wither skeletons killed, however, after killing 131 wither skeletons I have not received a single wither skull. I know this is possible, but statically it is extremely unlikely.