Unfair treatment

Went to Mr price rosebank mall to purchase a bag and was asked for ID and showed them my ID on the phone.the cushier told me that they want physical ID not on phone.,was confused because the ID on my phone is the same as my physical ID. Want to understand what exactly they are looking for in our ID. Felt like was treated like a ***** will I was using my account. And it was not the first time buying in this store or at any Mr price using my ID on the phone