why does the pip difference change in forex market?

In the United States, the FOREX market is a dealer market. If customers are taking all of the risk, then the pip difference can be small, but if dealers have to put their own money up, they will put a spread in the pips so they are not taking any risk in the transaction, the entire risk falls on the customers. It gives you a signal as to how liquid the market currently is.

Do note that in the United States, a FOREX dealer can quote any amount they wish without limitation. If a pip change could trigger a profitable margin call, they have been known to do that just for the commissions and to balance their risk book.