Kerouac, you are correct that curry powder already contains the spices which are then added again during the preparation of mix powder. The key point is that by adding those spices (mostly turmeric, cumin, coriander, but often some others in smaller amounts) you are changing the proportions of the overall mix. The spices I've mentioned, which are added to the curry powder, are the key flavours of BIR - turmeric especially so.
Consider for a moment that turmeric is already the dominant spice in any typical curry powder, and then in making mix powder you maybe as much turmeric as curry powder. The proportions of turmeric in the resulting blend are much higher, which gives you a clue as to how important a role this spice plays in the symphony of flavours known as BIR. Others which are added in large measures to mix powder, eg coriander and cumin, also play important roles.
If you were to only use curry powder to cook with, you would not have nearly so much turmeric, coriander and cumin in your dishes relative to the other spices which play more of a nuanced role.
BIR cuisine developed as a low cost "cheap night out", with first generation immigrant cooks providing cheap labour and restaurant owners who always had an eye on minimising the bottom line costs. It's likely they started bulking out curry powder with spices which could be bought more cheaply in bulk and by happy coincidence found that the flavours which resulted were accepted and came to be adored by the customer base. However and why-ever it began, bulking out the curry powders with extra turmeric, coriander, cumin and others is so standard in BIR that we should do it at home if we want to get close to real BIR flavours.