Bug prices in products

I have a serious error, as a visitor in the shop prestashop I find all the prices of the products well, when I decide to fill a cart I choose a pair of products, if the shipping data is from my country spain, everything is fine, but when that Same order I do it simulating with data that I am a buyer from another country prices go down, I do not find a pattern of error defined for same articles of the same category some go down and others are well, if there is a million tests to investigate the problem I I have tried three million, some help would be appreciated, because it would be easier to migrate quickly if it was a bug, but who can confirm it ?.