Not getting hole depth

Not sure if this is appropriate place to post this, but it is dealing with related forum topic content.

After an "air" test I proceeded and finally made my first hole but immensely disappointed it was not to the depth as defined in Fusion 360 2D Bore Hole Setup.  My post-processor is generic X,Y, Z mill for a PrintNC DIY build.

The depth of the hole should be -18mm depth from body top surface but I am only getting 8mm.  I am using a Genmitsu 3mm flat-end mill that protrudes from the spindle collet lock-nut 22.225mm.  Stock is 19.05mm MDF.  No offsets are defined - at least that I can ascertain.  Top Height is from Stock Top with zero offset.  Bottom Height is from Stock Bottom with zero offset.  The design is an extrusion with Start selected as Profile Plane, Direction is One, Extent Type is Distance, Distance is -18mm, Taper Angle is zero, Operation is Cut.

I zero X, Y, and Z on stock top.  The X and Y positioning of the hole is spot on.  The Z depth not so much.

Where could this 10mm "offset" be coming from?  Any pointers on where to look?

Thank you. 

What does the G-code file say?
Can you post the G-code file?

The bottom of the hole depends on the stock thickness, maybe that isn't what it is supposed to be?