Constructor initializer list and static member (private vs protected)

It is clear that a data member of a class declared as static is incorrect to be initialized in ctor initializer list (because it "is not a member" of an object). But it works fine if it is a protected member. Why?

// foo.h
class Foo {
public:
  explicit Foo();
  ~Foo() = default;
protected:
  static int kProtected;
private:
  static int kPrivate;
}

// foo.cpp
Foo::Foo() 
    : kProtected(1), // OK (?!)
      kPrivate(1) {} // error C2438: 'kPrivate': cannot initialize static class data via constructor