The difference is one is part of a larger, longer set of instructions while the other is a single sentence in a spot you can’t easily scroll past. There’s a reason tl;drs exist.
The number of people who cast an invalid vote in the last delegate election was two. The number of people who cast an invalid PM vote in the corresponding PM election was nine. Clearly this is a problem that has existed for quite a while, but it doesn’t seem like many people are repeat offenders.