Почему float.NaN! = Double .NaN в C #?

Почему float.NaN! = Double.NaN ?

в то время как float.PositiveInfinity == double.PositiveInfinity и float.NegativeInfinity == double.NegativeInfinity равны .

ПРИМЕР:

bool PosInfinity = (float.PositiveInfinity == double.PositiveInfinity); //true
bool NegInfinity = (float.NegativeInfinity == double.NegativeInfinity); //true

bool isNanEqual = (float.NaN == double.NaN);  //false, WHY?