Why does the Android Keyboard move words around?

Typing text messages on my Android phone is aided a lot by the 3 word suggestions that appear at the top. But it is extremely annoying that as I recognize one that I want and simultaneously finish typing a letter, I reach for the suggested word but it has moved to a different position! Then I have to backspace a bunch of times to undo, for example, 'necessary' to get back to 'new' - which is what I was reaching for.

Why don't the words just 'stick' in place once they appear? Who thought it would be good for them to move around while one is typing?