Ketchup Confusion

I've noticed that there often seems to be a moment of confusion when I ask for ketchup for my chips at the chippie or other fast food place, although I normally use brown sauce. The worker hesitates and asks if I mean "tomato ketchup" or "tomato sauce".

I was always a little confused why they needed to ask for clarification but never really gave it a whole lot of thought until I saw this article from the Middle Class Handbook. According to them, there is a pretty clear class divide in the British choice of condiments. Brown sauce is the middle class condiment of choice while "tomato sauce" is used by the working class. "Ketchup" is a more middle class term that the chippie worker would not use, hence his confusion when he comes across me.