Currency Trade Fill

I recently placed an order to sell 1 chaos for 30 exalt, a 1:6 on the currency market. This was for 5 chaos and one order was filled for 25 exalt. Not sure if this is a common thing people are experiencing on the currency market but this is something that I think should be addressed. When orders are placed they are usually for a specific price, a "limit order". Just for some peace of mind I would like an explanation that way I would have a better understanding of the situation for when I start needing more costly currencies like annulments and divines. Would a dev or anyone with more experience than me please explain how a "limit order" was filled by a "market order"? Is this just a bad fill, or is this a bug that just cost me 5 exalt and if it is a bug, is it exclusive to console? Thank you for your time.