I recently wanted a 3/4" wide by 1/2" deep dado. I installed a 3/4" bit in the table-mounted router, positioned the fence where I wanted and ran the piece through. The dado came out fine except the following: The bottom of the cut was about 1/16" higher on both sides than in the middle of the cut. The middle of the cut was 1/2" as desired but the depth on both sides was about 7/16". I think the speed was set at 15k rpm. I had to chisel out the rest to make the dado even at the bottom of the cut.
Any ideas about what went wrong?