From the wiki:
The orders are also placed in a FIFO system--whoever placed their order first will have theirs filled first. If a country's order is filled before the full amount of items on the market is bought, the next standing order will then be filled, or, if there are no more standing orders for that item at that price, the remaining units will be made available for others to buy manually. However, if a country's standing order is interrupted, such as by a lack of cash on hand, that country's order will then be placed last, and the next standing order will then buy whatever is remaining, and so on.

So just because you place an SO for a higher amount than someone else doesn't mean you get them. However, if your SO is $1 higher than the current high SO and someone puts goods on the market at your price, but not below, you will get them.