Because I am from the UK and consider myself a UK citizen. Asking "where I'm really from" comes across as if I'm not of this country. It's just tactless and needless. If you're really interested in my family heritage then ask that. Don't ask where I'm 'really' from because shockingly enough, I'm *really* from the UK.
Fair enough. I know from personal experience that my mum is scared of being accused of being racist so she quite often says things that are not considered PC nowadays but were at one point the "nice way" of saying thing. I tend to try and take things the older generation say in a positive light unless its clear they are trying to be nasty.