Here's an example to illustrate the point: You buy 5 beers in a bar advertised at 40 pesos each. This bar offers an exchange rate of 10 pesos to the dollar. You could either pay 200 pesos or $20.00, fair enough.
Next door, the bar is selling the same beer at the same price of 40 pesos but they have an exchange rate of 11 pesos to the dollar. Your same 5 beers, will still cost you 200 pesos but pay in dollars and instead of $20.00 it's $22.00.
In the 2nd example, 200 pesos at 11 pesos to the dollar would be about $18.18, not $22. The way I see it, you could pay with a $20 bill, and since the site says you'd probably get change in pesos, I figure that you'd get maybe 15 pesos in change (rounding to the barkeep's favor).
And in California we have people whining because 8th graders are going to be tested in algebra.