I cut the first leg mortise in the slab for my new ash workbench and I’ve hit a snag.  I can’t figure out what the problem is.

If the reference surface on the leg is flat and true…

Which is it.

And the mortise is cut into the undreside of a benchtop in a plane that is perfectly flat…

Which it is (double checked it with my square).

And my shoulder line for the tenon is perfectly square to the reference face on the leg…

Which it is (sorry for the blurry picture).

Then why is the reference face of the leg not perpendicular to the underside of the benchtop when the shoulder is perfectly seated to the leg?

What am I doing wrong?

My best guess is that there is some optical illusion from the toothed surface and the tenon isn’t actually seated all the way.  I can bend the leg out a bit to make it square, and it’s 12/4 ash, so that’s probably at the joint, not the leg itself, right?  But I fear that adjusting the inside of the mortise might make the tenon loose (it’s friction fit right now).

JPG