In raw mode the post history dialog interleaves the left and right versions, preventing you from easily copying the text of a particular version.
I’m guessing this has been designed this way to ensure the diff sections line up, but I’m sure there’d be another way to design the HTML such that you can select all of one revision without including part of the other (though it would probably be much more complex than the current HTMl’s table! Maybe the CSS grid model could help?)