The Ramban (Baba Metzia 82b) proves that the rule of Adam muad L'olam bein B'oness bein b'ratzon (sanhedrin 72a), [freely translated: A human is always liable for damages, whether the damage was done unintentionally or intentionally, whether by unavoidable accident or whether it was done willingly.] is applicable even by Oness Gamur, (completely and toally unintentional), from the Gemara (Baba Kama 27a) which says "נפל מראש הגג ברוח שאינה מצויה והזיק חייב על הנזק -, If one fell from a roof due to an atypical wind, such that it could not have been anticipated in advance that he would fall, and while falling he caused damage, he is liable for the damage. (Tosfos disagrees with Ramban).
The Chelkas Yoav (vol.1:1) says that the reason why a katan (minor) is not punished for aveiros which he does is since he is unresponsible for his actions he is considered an oness (acting unwillingly).
If so, why, according to the Ramban, is a katan not liable for causing damages to others, according to the above ruling of the Ramban that even an oness gamur is liable?