Looting 3 doesn't affect shulker shell drop rates properly

While end rating, i've realised that lotting 3 didin't increase the drop rate of shulker shells when using fire aspect. After some tests in a creative world, iv'e come into this conclusion. Even though i'm not sure due to the inconsistency of the effect, every 5 shulkers killed with ano looting, looting 3 and lottin 3 + fire aspect swords, the drops were 4, 12 and 5.
Please check this out, end rading is already difficult enough.