It's a magical world

It seems to me, though, that the entire point of both manga is, on some level, "negating what already happened"--Yuuko's lines in this chapter about straightening time out certainly seem to imply that, when the action of TRC and Holic has finished, time will have been straightened out: I think that people in Watanuki's world are starting to forget Yuuko is a sign that the process is already happening. Which is why it's so important that Doumeki find the proper moment to use the egg, because when time straightens out, Watanuki's existence will be erased, since he came into being as a consequence of things being twisted.