This sounds suspiciously like trying to make people do math

Because that way you’d get people asking for unholy abominations such as gravy and curry sauce, tuna and beans, or coleslaw and bolognaise.

Let’s assume the currency is £ (but that doesn’t really matter for the example).

Chips = £2.20

Chips + Cheese = £3.00

That means, that just the Cheese = £3.00 - £2.20 = £0.80

Chips + Beans = £2.50

That means, that just the Beans = £2.50 - £2.20 = £0.30

Chips + Cheese + Beans = £3.50

£2.20 + £0.80 + £0.30 = £3.30

I guess they did this so people don’t notice they are getting ripped off…