Lets say I open a first position for 10usd then second position I want to open is 20usd (while the first positio is still running)
I select 20 usd in the exchange for the second position and the exchange opens a position for 17 instead of 20 for example..
I am using limit orders and is not related to the minimum amount to be traded
This is how exchanges are. You will noticed that the amount you use to oron a position is also changing. Sometimes I it will increase and sometimes it will be decreasing but depending on if you are making money already or losing. You do not need to bother yourself about this. Just use the amount to open position and take note of your profit and loss.
An example is when I open $250 long position, I can see $247 instead. Do not worry about it. I do not know the reason exchanges make it like that though.