why can't ios::ate and seep(0, ios::end) in open() cannot serve like..

[hughLg]

outF.open("file.dat", ios::out | ios::binary | ios::ate);
	//outF.seekp(0, ios::end);
	outF.write((const char *)&num, sizeof(num));
	num = 3;
	outF.write((const char *)&num, sizeof(num));
	outF.close();

Why the open("file.dat", ios:ut | ios::binary | ios::ate) and seekp(..) cannot make the append operation success?

It always write from the start of the file! Yes, I know it can easily be done with ios::app in open function, but why?