Fun fact: Unlike Int, UInt doesn't extend Number, but sometimes it does

I bumped into this interesting behavior around unsigned types. At first I was confused why UInt doesn't extend Number, but as I was experimenting, it got weirder. It sometimes is a Number (see case c below) and sometimes isn't (see case d)?!

val a: Int = 12
a is Number // true

val b: UInt = 12u
b is Number // doesn't compile, `Incompatible types: Number and UInt`

val c: UInt = 12u
(c as Any) is Number // true, Idea says "No cast needed"

val d: Any = 12u
d is Number // false

I guess this is partly legacy of Java, which doesn't have unsigned types, and partly effect of UInt being value class. I'm curious if someone has a deeper explanation.