Great question, Jan. In addition to variances in the thickness of available wood, I have found out the hard way that there can be variances in the router bits themselves.
I have four 3/8" (.375) straight bits made by a major manufacturer with a great reputation. Same model number and everything. They mike out at .371, .374, .378, and .379. I found this out while making box joints on the router table. I thought that maybe my measuring ability, or my cheapo micrometer might be the problem, but I took them to a machinist friend and he got the same measurements. So save time, aggravation, and money in the long run, and make an exact width dado jig. Jim
It seems I never finish what I