On the other side it is rational if you think about it in abstract where there is no latency between screen and keyboard input.
The likely reason you continued typing is that the prompt didn't include what you wanted.
I can see it as rational from a number of viewpoints - "When user types 'fo' they want to go here, 'foru' usually results in this, 'forum' has this result etc etc.
But I think all those other viewpoints lead to less desirable behaviour, for me. I'm not going to pretend it's universal because I don't have the research, but it pisses me off.
