Waddy's answer is a good one but not the correct answer in this case. The truss rod is engaged with a slight degree of back bow. I do see how Waddy could be right but the major cause of the slots being lower than the fret height follows:
Jeff is correct - congrats Jeff!!!
With an angled back headstock (this one is 15 degrees) the strings are ramped upward to the nut slots. When the nut ends at the face of the nut and the strings leave the slot the strings will no longer be at the 15 degree angle and instead will conform into the direction and plane of the saddle + nut slot. But... right where the strings leave the nut slot there is a bending that occurs in order to change the angle to now meet the saddle. This very slight bend is why nut slots may be cut below the fret height and still not cause any buzzing.
The composition and stiffness of individual strings will impact this too. In the case of the nut pictured all of the nut slots are cut correctly. In fact a couple of them could be cut a thou or two deeper but I chickened out since at 52 years old I can't see shit anymore...
Now if you understand what I just wrote above and translate this thinking to the case of the zero fret you can see how this very same thing happens there too although perhaps not to the same degree because the angle behind the zero fret at the head stock may not be as much as 15 degrees and or the nut distributes the transition from the headstock angle to the fret board angle in conjunction with the zero fret. Whew - that was a long sentence...
The first time I noticed this I initially thought that I had cut the slots too low or the slots were imperfect and the string was not sitting at the bottom of the slots. I asked David Collins if I had screwed up, again...
, and he explained the above to me. It was a teachable moment but without a screw up...
Thanks everyone!
