Using nil-coalescing operator with try? for function that throws and returns optional

I want to use nil-coalescing operator to set a default value in both the following cases:

1. function throws an error
2. function returns nil

Please take a look at the code snippet below. I have the following questions:

1. Why is item1 nil?
2. What is the difference between the initialization of item1 vs item2

enum VendingMachineError: Error {
    case invalidCode
}

class VendingMachine {
    func itemCode(code: Int) throws -> String? {
        guard code > 0 else {
            throw VendingMachineError.invalidCode
        }
        if code == 1 {
            return nil
        } else {
            return "Item #" + String(code)
        }
    }
}

let machine = VendingMachine()

// Question: Why is this nil?
let item1 = try? machine.itemCode(code: 0) ?? "Unknown"
print(item1)
// nil

// What is the difference between the initialization of item1 vs item2
let item2 = (try? machine.itemCode(code: 0)) ?? "Unknown"
print(item2)
// Unknown