I have a feeling that competition is fiercer in the US, which I believe is the largest film market today.
In addition to sales tax, there is also the matter of how much it costs any manufacturer to sell their film in specific countries. How much they are taxed on their sales to distributors, etc.
Fuel costs for transportation, cost of inventory, turnover, etc.
You may also wish to consider purchasing power parity between individuals of the countries.
In combination with the rules of supply and demand, I'm sure that answers a lot of the questions.