So there is an unintentional taper? Maybe you can double check the angle of the top slide. You can maybe run a dial gauge along the side of the slide to check where it is tilted towards, then use a hammer and lightly tap it until it's zero-ed.
EDIT: a 0.6mm deviation is huge!!!
I blame my machining process...
Start with 12mm thick cylinder, stick it into lathe.
Face off side A.
Take out, flip, and face off side B. Bore the hole with drill bit and reamer in the lathe tailstock while it's in this position. Obviously if I did this right, side B should be nice and normal to the hole. Side A at this point is probably not normal because of chuck errors and stuff.
Then like the doofus I am, I think I loctited the cylinder to the steel shaft with side A facing in towards the chuck, and side B facing the cutting tool..
I then faced off side B until I measured the thickness at 4.5mm. but the rest wasn't 4.5mm....
So I think that's what went wrong.