Dr.Sze understands what we talk about, sure.
The LM399 is a mass produced cheapest burried zener Vref today, afaik. ADI is not messing too much with it, imho, they anneal once, they test the heater and 6.9V output, when ok the chip pass. The production costs are in XXcents levels (ie. the yield 90%). The chip is intended for general use in the industry and with mainstream meters till 6.5digits where the few uV popcorn does not mean much (moreover it is a "high noise" device, perhaps because of wide spectra of popcorns as Kleinstein mentioned above).
The production process of the LTZ1000 and LTFLU is different, imo. They care much, may be they do several special additional processes after the ion implantation, and provide careful tests. The yield could be much lower (easily 5-10%), therefore the cost per chip could be is ZZx higher. It does not mean the LTZ1000 or LTFLU is popcorn free by design ,however.