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This is probably ‘change’.
If I create a new wallet and receive 1 BTC, my balance is 1 BTC.
If I then send 0.1 BTC, I actually send 0.1 BTC to you and send 0.9 BTC back to myself as ‘change’. Wallets typically generate a new receiving address in order to send the change back to itself.
Each input into your Bitcoin wallet has to be spent in it’s entirety, like a $20 bill. You don’t tear a piece off when you spend $5.
You spend the whole $20 and get $15 back into your wallet.
Bitcoin works like that.