They assume that us european barbarians can't tell Chinese women apart.
There is something in that.
They all have black, straight hair, brown eyes, normally pale skin, and within a certain body type can therefore look very similar.
I'm sure a Chinese can much more easily recognise another Chinese that Western eyes could. Western eyes that are used to many different-coloured eyes, different colour hair, curly, straight, different skin types etc. The variation in Western faces is much more than with Chinese. I realise that there are regional variations in looks in China, but we always seem to get the same type of Chinese girls here.