There's also sometimes an element of "I don't care about that stuff, I want you to deal with this thing over here instead" - e.g., "Don't worry about spelling, I'll clean that up later. What do you think of the plot?". Even if the criticism is correct, irrelevant criticism can reduce the relevant information available. This can actually make the bucket problem worse in some cases, such as if you spend so long editing spelling that you forget to talk about the things that they did right.
The best way to split someone else's buckets is often to give explicitly different comments on different parts of the bucket, to encourage a division in their head. You can even do that yourself, though it takes a lot more self-awareness.