In my example (same as in the lecture) the wallet 1 starts the auction and wallet 3 wins it. As the contract always has to have some ADA as Lars explains it looks like Wallet 1 pays 2 ADA to the contract in slot 1 to satisfy this rule as it has 2 ADA less (minus fees) in the UTxO. See:
But in the end Wallet 3 who wins the auction gets this 2 ADA. I guess those 2 ADA have to be there to cover some fees in some special cases. But why does Wallet 3 and not Wallet 1 gets the 2 ADA in the end?