Get real fee per transaction I am trying to get real fee per trasnaction.

So I have an account whit balance 0.06940000 and have a second account with balance 0.00000000.

To get current fee I have run the command

bitcoin-cli getmempoolinfo


and got the mempoolminfee = 0.00001000.
I thought that when I send the 0.06939000 in my account rest balance will 0.00000000. but I got 0.00004860. Actually the fee was  0.0000514.

general question is WhY ? 

Please tell me to understand how is fees working!
What getmempoolinfo is returning is not the minimum fee to send a transaction, but the minimum rate per kilobyte to send a transaction. Your bitcoin client will take the configured fee and multiply it by the size of the transaction in kilobytes to calculate the actual transaction fee.

The size of a transaction is somewhat difficult to compute (for a person) because it requires understanding the number of inputs that are required to fulfil the transaction, the number of outputs (primary recipients + change address), and the targeted transaction fee (since appropriate fees could eliminate the need for a change address).

There are online fee calculators out there you can use to estimate these, but it is still difficult to completely empty a wallet with a known fee using the straight bitcoin client.