Well, the reason is quite simple, because taxes changed since the volumes were published and will change again. ^^
When you buy a japanese publication you can find two prices, one in bolded font and another inbetween breaks. Well, that one is the original price, and the one in bolded font is that price with the taxes added, if the taxes change, when you buy a japanes manga for example, they will use the ammount inbetween breaks and will add the tax ammount that is valid actually. That's why I list the prices like this. ^^