Fuse question

I'm wiring in my Baja Designs off-road lights. I want to fuse the low current side of the relays.
So I bought an add-a-fuse and planned to use one of the fuse locations under the dash by the drivers left knee. I also had 2 criteria. That I was tapping into a non-critical fuse, and that
the fuse got power only with the ignition "on". So looking at the fuse descriptions, I picked the drivers side rear power window fuse. A 20a non-critical fuse that only gets power when the ignition is on. Supposedly in position 13 of the fuse block. So I pulled the fuse, and then as a double check, checked all the power windows. To my surprise, all 4 power windows worked fine. How can this be? The 20a fuse for the drivers side rear power window is removed?
I've stopped the wiring at this point because I'd like to be sure that the fuse I'm tapping into is what it's supposed to be. Any ideas?