当创建数组的方式不同时，反初始化顺序也不同

TL; DR：为什么在将数组与初始值一起创建时，立即取消初始化值，但是在创建数组后填充数组值会产生不同的行为吗？

我在具有GC的JavaScript之后学习Swift，因此ARC和反初始化的概念对我来说还是个新手。为了更深入地了解它，我根据官方文档中的示例编写了以下代码：

// definitions; see the next two snippets for usage
class Bank {
    static var coins: Int = 100 {
        didSet {
            let diff = coins - oldValue
            let sign = diff < 0 ? "-" : "+"

            print("Bank: \(oldValue) \(sign) \(diff.magnitude) = \(coins)")
        }
    }

    static func give(_ amount: Int, to user: User) {
        guard amount <= coins else {
            print("[ERROR] The bank does not have enough coins; requested \(amount - coins) more coins than available")
            return
        }

        user.purse += amount
        coins -= amount
    }

    static func take(_ amount: Int, from user: User) {
        user.purse -= amount
        coins += amount
    }
}

class User {
    private static var count = 0

    let id: Int

    var purse = 0

    init() {
        User.count += 1

        id = User.count
    }

    deinit {
        print("User #\(id) perished")

        Bank.take(purse, from: self)
    }
}

当我使用现有的用户实例（array = [value, value]）创建数组时，取消初始化会在分配给nil（array[0] = nil）之后立即发生：

var users: [User?] = [
    User(),
    User(),
]

Bank.give(90, to: users[0]!)
users[0] = nil
Bank.give(50, to: users[1]!) // works

//  Bank: 100 - 90 = 10
//  User #1 perished
//  Bank: 10 + 90 = 100
//  Bank: 100 - 50 = 50

…但是当我首先创建一个空数组，然后使用值（array = []; array[0] = value; array[1] = value）填充它时，稍后会在某个随机点进行反初始化：

var users = [User?](repeating: nil, count: 2)

users[0] = User()
users[1] = User()

Bank.give(90, to: users[0]!)
users[0] = nil
Bank.give(50, to: users[1]!) // fails

//  Bank: 100 - 90 = 10
//  [ERROR] The bank does not have enough coins; requested 40 more coins than available
//  User #1 perished
//  Bank: 10 + 90 = 100

从编译器的角度来看有什么区别？造成这种差异的理由是什么？