Actually - while the ACTIVE ingredient in cheaper brands of medications are the same, the OTHER ingredients can be slightly different, or different quantities.
I find that as I have allergies, I have to be very careful which brands I stick to.
For example - I can't use any type if soluble medications, as I am allergic to the binder that dissolves. That goes for OTC supermarket brand soluble paracetamol just as much as it does nurofen meltlets...
So there ARE subtle differences that most people don't notice, but will work slightly differently.
I have an intolerance to magnesium stearate, for example - and slight differences in the quantity of this in medications can mean the difference between me having a reaction or not having a reaction. Try finding ANY pills without magnesium stearate in as a filler, and then try looking at the differing amounts in 'proper' branded nurofen and then a supermarket own brand.
In that case, at least in Tesco, the supermarket ones actually have slightly LESS magnesium stearate in.
It's not as clear cut as you think, for some people, OP!