Banknotes of certain countries are of more interest to the average American collector than, say, his British counterpart. Vietnamese notes, for example, are more popular in the USA, due to the war it fought there. There is evidently more demand in the USA for expensive notes - especially, of course, expensive American notes. For one thing, there are many more wealthy collectors in the USA than elsewhere. When a British dealer chances upon a high-valued foreign rarity, he may offer it at below 'Pick' value if he wants to make a quick sale within Britain.
Some banknotes are inevitably easier to obtain in the USA than in Europe.
Others are harder to obtain there, and the American collector will have to pay more for these than he would in the continent of origin. To some extent, this effect is balanced out by the fact that a country tends to be more interested in the banknotes of its neighbouring countries than anyone else is. So although the demand is higher there, so is the supply. Compared to most countries, there are more dealers in the USA, and in particular, more dealers with substantial buying-power. If you are a collector in the USA and you want a certain note, you have a better chance of finding a dealer who has been able to buy a quantity of that note.