This is my first time travelling to London and I'm very confused by the travel infrastructure here. When I landed at Heathrow, I bought a 7 day Oyster card for Zone 1-2 and I was charged £43 for it. After I took my journey to Central London and I saw the balance, it said that it had deduced £1.5 and my remaining balance was just £3.5.
Where did the rest of my £38 go?