When I top up my Oyster card at a Southeastern ticket machine, I can specify the precise amount of my top-up in pounds and pence.
This is how I ended up with the odd 1p on my card. When they were giving out free Oyster cards without a deposit they had a nominal 1p credit, I believe due to system limitations in activating the cards.A number of years ago I was sent a PAYG card by post, that came with 1p credit.