Some ceiling fans in my house have 12/2 Romex ran to them and some fans have 14/3 Romex ran to them. Why the extra conductor to certain fans?
The 3 wire will allow you to have control of a light and fan seperately.
and sometimes, they just grab whatever wire they have for a short run but, most likely forward planning like nealtw says.
Someone might of replaced a light with a ceiling fan. Technically ceiling fans should have rated boxes.