Why in yaoi manga it's that surprising when someone cum dry? Care to explain?

because it is rarely that a men can come without touching their dick I think

No no no no, this is a very special thing!!! I first found out about it in the movie Go, and then years later managed to do help my partner do it, just the one time. Check it out:

http://www.whitelotuseast.com/EjaculationControl.htm

didn't misono have a prostate orgasm though ?

It is when the semen is forced in the wrong direction and flows back into the bladder. Later to be released through urination.