Question about isPlainObject method

Question about isPlainObject method

from the source code: "return typeof Ctor === "function" && toString.call( Ctor ) === ObjectFunctionString;"
jQuery .   isPlainObject   (   {}   )   // true  
jQuery .   isPlainObject   (  new Object   )   // true

but  i don't understand this situation:
/*
function Ctor(){
}
jQuery .   isPlainObject  (new Ctor ); // true
*/
because toString.call( Ctor ) === ObjectFunctionString // this should is false.but result is true