iPhone 5s sim confusion

So, we were recently gifted by a family friend an iPhone 5s who apparently bought one for us and for himself from dubai (we live in india) from some bulk dealer. We received it perfectly packed and it was warrantied too. But when i opened the sim tray, i was shocked to find an at&t nano-sim inside. Just to cross-check i checked the model number, and it was A1533 which is the one of the unlocked model. Then what could be the reason for this?