Silly physics-defying router...

So I was working on some pieces for the bottle rack last night and determined that my router is defying physics. Not sure how that's going down, but it clearly needs to stop.

Here are some diagrams showing you the various aspects of the router and the jig I made for it. I'm hoping someone will see something I missed and be able to explain it to me.

This is the bottom of my router. It's a 6" diameter circle with a 1/4" bit in it. The distance from the edge of the face plate to the edge of the router bit is 2 7/8". That works out, both math wise and when I measure it.

This is the piece I want to make. I want a 3/4" wide groove 1/4" deep all the way across this piece.
This is the top view of the jig. The top and bottom pieces (with the dots on them) are screwed to a piece of particleboard. The two vertical pieces on the sides are on top of all three of the horizontal pieces of wood in the middle. They are the stops for the edges of the router.


This is a side view of the jig. The dotted line shows the part I want to have removed. These measurements should, by my thinking, give me exactly what I want. it just isn't working that way and I don't know why.

What I want to do with this is to create a groove for the middle shelf. It will look cooler than a butt joint and will be stronger as well. I just don't know why it isn't working for me. I'm going to have to work at it again tonight. Thankfully I ruined a few perfectly good pieces of wood by getting it wrong so I don't have to go purchase scrap.