Langston picked out a point I avoided. WPM on typewriters was key way back when, because a typo was really hard to fix. Even when that neat new invention, "Liquid Paper" (I'm not that old, am I?) came out, it was still an issue when filling out carbon-copy forms (no, I'm NOT that old, it was a high-school summer job).
With computers, all that went out the window, because with one click your typo is gone. Master the abstract concepts of insert vs. overwrite, and editing typos is a breeze. I have no idea how you'd factor this in to calculating WPM, something I never did factor for myself and wouldn't know how to now.