It sounds like you are being confused by the change output.
Here is what is happening:
-Lets pretend you receive 1BTC to your address X
-You send 0.6BTC to address Y
-What Bitcoin Core does is it sends 0.6BTC from address X to address Y, then it generates a new address, called a change address, and sends the remaining 0.4BTC on address X to the change address.
The purpose of this is to increase the privacy and security of your transactions. It makes it more difficult for someone to track your coins.
This is a basic explanation, it actually works different to this. More explanation here:
https://en.bitcoin.it/wiki/Change