Inverse grep does not find all matching lines

**EDIT:** I am making this too complicated. It's not about inverse grep. I get the same results using just grep -x -f stop.txt < in.txt. If who comes before whose in the stop word file, the result is just who. When the order is reversed in the stop word file, both lines in in.txt are found. I have the feeling that I fundamentally do not get grep. *** I cannot get inverse grep to work like I expect in order to remove lines containing a stop word from a file. The order in which the stop words are given affects the result. Suppose I have two files. An input file in.txt:

who
whose

And a file with a list of stop words stop.txt:

who
whose

If I "filter" in.txt with an inverse grep search on the stop words in `stop.txt`, I get:

$ grep -vx -f stop.txt < in.txt
whose
$

Only if I change stop.txt to

whose
who

do I get:

$ grep -vx -f stop.txt < in.txt
$

I do not understand why the order of words in the file with stop words is of importance.