Implicit type conversion between uint8 and byte

As said in The Go Programming Language on page 55: "an EXPLICIT conversion is required to convert a value from one type to another", even if both of them have same underlying type. For example:

type myByte byte

func main() {
    var a byte
    var b myByte

    a = b       // Compile error: cannot use b (type myByte) as type byte in assignment
    a = byte(b) // OK
}

But for uint8 and byte, I'm surprised that the conversion is implicit:

func main() {
    var a byte
    var b uint8

    a = b // OK
    b = a // OK
}

So why?