Why is the SE editor's undo function so unpredictable?

Question

When you hit Ctrl+Z, the number of inputs you've entered that are undone seems pretty random.

In the latest example that brought me here, I was converting regular lines into a blockquote in this question,¹ and had done about 20 of them by this method:

1. Type two spaces at the end of a line and copy them
2. Hit ↓ and End
3. Hit Ctrl+V
4. Repeat steps 2 & 3

I accidentally forgot to hit End in a line, pasted in the wrong place, hit Ctrl+Z, and all of the double spaces were removed up to the start of the blockquote. Argh!

But there are many such instances. As a case in point, I just wrote the first sentence of this paragraph, hit Ctrl+Z, and the whole line was removed. But if I select a random word in this paragraph and type another in its place, and then hit Ctrl+Z, only that one word is reverted to what it was. The problem is that when behaviour is unpredictable, users avoid the function. That's what I do whenever I consciously think to myself, "Oh, right, I'm using the SE editor. There is no reliable undo."

I know this isn't unique to Stack Exchange — Word also tries and sometimes fails to guess a more "natural" delimiter of your sequence of inputs — but I find it particularly bad here.

So why is it like it is, and should it be changed to a more standard algorithm?

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_{¹ Unrelated, but entering this link reminded me that on SE, unlike every other site and word processor, adding a link is Ctrl+L instead of Ctrl+K, because the latter is used for code insertion...}