There are the big order at the lowest price. This order get huge amount of hash power for very cheap. It can increase the price untill other orders move the price up, then it can jump down to its cheap point while other orders can only step the price down once every around 500 sec. This is unfair.
Every real time market has rules, you just have to learn how to use the rules to get what you want.
I am pretty sure all orders have to follow the rules, you should check the order numbers when you see a large jump down.
If orders could be changed down to any price, the price to the sellers would always be the minimum and that would not be fair to the sellers.
Without sellers nicehash would not exist.
You can easily change the price down by more than the minimum but creating a new order at the lower price and cancelling the higher order.
But keep in mind you must pay the order fee on the new order, currently .0001, so consider that in your calculations.