Financial questions that need an answer

Well.. everyone I know got their number through automatically.. but my mum got a letter through asking somehting along the lines of what study my mum would like on it or something..? As my parents aren’t married (they are together) I have my dad’s surname but my mum is the legal parent if you know what I mean
Anyway… I was wondering why they were asking this.. Why would I suddenly change my surname to my mum’s surname? Anyway.. don’t they just take the study off child benefit? We’re very confused.. and my mum is going to phone up the NI people.. just wondering if anyone had a similar problem.