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…