A mistake non-framebuilders make is assuming bicycle tubes are straight. They seldom are straight. In my frame building classes I have my students roll the tubes on the alignment table to find and mark where the bend in the tubes are so the miters will place the curve in the plane of the frame. It is common for them to be as much as 1/8" out. The reason they are not more accurate is because they would cost a lot more to make.

While we are marking curves we also mark where the butt transitions are located. These can be pretty far out of spec too. Both of these references let us know where to best place the miters. These are examples of how a well made custom frame can be made in a superior way to production frames where the factory workers aren't looking for individual variations.