Unexplained Segwit transaction results in wrong balance (mixed address transaction?) I sent 0.005 BTC from my Ledger Nano wallet (Segwit) to my Bitstamp wallet.
When checking my new balance on a public address reader, an additional 0.603 BTC was removed. I then checked the transaction online on blockchain.info and the 0.603 BTC went to an unknown-to-me address (I never authorized nor performed this second transaction). 
My Ledger Nano wallet (Segwit) balance on my Ledger Desktop app always showed the correct balance (with only -0.005 BTC) when the other sites all showed the incorrect balance ( -0.603 -0.005 BTC).
I finally transfered the total remaining balance from my Ledger nano to Bitstamp as a test, and I got back the 0.603 BTC missing.
What is interesting is that the total of the transaction (authorized
0.005 + unauthorized 0.603) is equal to a transaction I have done two weeks before (from an exchange to my Ledger Nano). 


Can someone explain to me what happened? By not understanding what happened, I am fearful of other unexpected transactions and wonder if I should still use my Segwit address.

Thank you
You just experienced what is called a change address, when you send a transaction from an address, the wallet sends the amount to the address you put and sends the remaining balance to another address in your wallet. this is why you saw the correct balance in your ledger because the address is still in your ledger.