It only modifies the historic line IFFF you don't hit ENTER before leaving that line.
echo "miss daisy"
Hit up arrow, add ' crazy' after daisy, and hit ENTER.
Use up arrow to look at history -- both lines are intact.
Pick one, edit it, then use up or down arrow to move away, then come back, and you will find your edit intact. Hit ENTER.
Use up arrow again, and you will find the modified line, not the original line.
Near as I can guess, it only does this because the alternative is that if you edit a line and move away, perhaps to look at a different line to remember what was typed, then move back to finish your edit, your edit would have been discarded. I think this would be just as annoying as corrupting history, but at different times.
I can't say as I like it or despise it; it's a "feature" I take advantage of, sometimes happy to have it, sometimes annoyed at not having a true reliable history of what was actally executed.