swift对齐问题withUnsafeBytes和UnsafePointer

Question

在以下代码中，testP.deviceID和testP.deviceID1应具有相同的值。但是testP.deviceID1包含2（它应该包含513）。所以有一些基本的东西，我不理解对齐？

let byteArray:[UInt8] = [ 0x01,0x02,23,24,25,0x01,0x02 ]
let testData = Data(bytes: byteArray )
struct testStruct {
    let deviceID            :UInt16 //0..1
    let byte0               :UInt8
    let byte1               :UInt8
    let byte2               :UInt8
    let deviceID1           :UInt16 //5..6
}

let testP = testData.withUnsafeBytes { (ptr: UnsafePointer<testStruct>) -> testStruct in
    return ptr.pointee
}

print("\(testP.deviceID):\(testP.byte0):\(testP.deviceID1)")