Going from 4 to 5 seems to me like silently changing “if A then B” to “if B then A”. Which is a logical mistake that many people do.
More precisely, it is a silent change from “if NULL, then DATA with very low proability” to “if DATA, then NULL with very low probability”.
Specific example: Imagine a box containing 1 green circle, 10 red circles, and 100 red squares; you choose a random item. It is true that “if you choose a red item, it is unlikely to be a circle”. But it is not true that “if you choose a circle, it is unlikely to be red”.