Just because your wi-fi is present, it doesn't mean it is always on. Sometimes it may drop out and then re-connect, and you wouldn't know this if you are sleeping. During these periods your phone would revert to using your 4G data connection and use your data to update while the wi-fi may be down even for a short time.
It's just a possibility .. you can't tell me you manually turn your your mobile data off as soon as you get home .. do you ?
It's not the first thing I think of when I get home, surely you must forget once in a while ?