It does care. Because if there is no data in one partition, then the result varies.
That was my understanding, but the supplied answer seems to say that the function will return a partition number even if that partition has no data. To my eye, $Partition.PF(Key) needs Key values to be supplied if it is to return partition numbers for those Keys.
And can we stop talking about the missing Group By, and concentrate on the meat of this question? Please?