Brian
The issue is that both fences are off 45 DEG. your frame mitres work good and that indicates that the fences make up 90 DEG.
On my sled I can compensate in two ways I can set the fences to a perfect 90 DEG. by loosening the bolts and adjusting it and also rotate the complete fence assembly to get a perfect 45 DEG.on both sides .
I would make the fences on yours movable and use a good drafting square to align them to the blade body not the teeth .Also a stop with a dowel attached may help you to get better precision as you are not aligning on the tip of the mitre but somewhere on the face of the mitre .