If the autocorrect were implemented in any other way it wouldn't work. It wouldn't allow you to type as quickly as you'd have to constantly pause on each word to see whether you'd typed it correctly or not, then make a manual correction. In other words, you'd have to know you messed up.

When I use the keyboard I just keep typing. I generally have no clue whether or not I've mashed the wrong key because I'm concentrating on the keyboard and not the part of the screen the test is appearing in, unlike a normal keyboard/screen relationship on a computer.