Time travel. Things happened that shouldn't have, but then did, without any of that erasing business. So change is possible. But erasing...
[Ponders this for a minute.] Well, think of it this way.
Something simple. Like...when you write on a piece of paper, but you wrote the wrong letter. You try to erase it, but no matter how much you erase, there's always some faint mark. You can write over it, but it's still there, isn't it?
But let's say you were making an L, but wanted a B. It's easier to make that L into a B and no one would be able to tell it was there. Because it was just changed, not erased.