A user might refer to the ETH amount on top of the screen and think that he/she has enough ETH to cover distribution costs.
In reality the user should look at the AVAILABLE amount of ETH in the wallets page to determine the actual amount of ETH.
The picture above explains more clearly what is going on.
The top part of the screen shows the total amount of ETH available to the user.
But in the AVAILABLE column, you could see that the available amount of ETH is actually less than what is required to distribute ( 0.001 ETH ). There is 0.5 ETH in orders; which is not counted towards the cost required for a user to distribute.
So,in order to ensure that the user is able to distribute correctly, the AVAILABLE amount of ETH has to be more than 0.001 ETH.