Digital signatures are always different because they are based of user private keys and transaction details.
A user's wallet is not like a regular wallet, it does not store coins inside. A wallet is a public key, an address in the network.
A wallet is a collection of transactions, associated with your address.
So when you are sending BTCs to someone, the system checks your history of transactions to see if you really have enough BTCs transferred to you.
And since each transaction has a unique identifier, transaction details are always different, which gives a unique digital signature.
So if you are still not sure how bitcoin transaction work, read this article:
https://finrazor.com/news/bitcoin-transactions-how-do-they-work