How is the DEX price determined

So i created an order to buy 50 XCP on the dex.

The dex determined a price of something around 0.0036. I just went with the standard parameters.

However when i checked the order book, the lowest sale price was 0.0041. Hence i do not understand why the dex comes up with this 0.0036 when it should be 0.0041?

As a user i guess this means I should not use the standard price.

Tor the counterparty devs i guess this means: adjust the price so that if i ask the dex to buy 50 XCP to give me the price that i need to accept in order to get the XCP right away. I.e. the price should go up if i buy more.