That is because they have changed their withdrawal fee (to 0.0008 i think).
And now they are charging it backwards for the last transaction you did.

But you can also go in negative balance again.
Means you now are -$4, purchase something, get $10 and have $6
on your wallet.
NOW you can send these $6 dollars and go in negative again to not
completly burn all of your funds

Pretty fraudulent in my eyes, but thats how they do that