QTR-Quadtone RIP

I recently switched my Epson 4880 printer from MIS matt black to photo black.  

I noticed that the MIS photo black showed "gloss differential" -- i.e. deep black showed a slight "haze" in comparison with other tones and colors.

Since the 4880 is sitting next to a 3880, which is using Epson K3 inks, I printed out a page of pure black on the two printers, using Kirkland glossy paper.

The Epson black was plainly glossier (no haze at all).  I measured the densities:  Epson photo black had a density of 2.48, and MIS photo black had a density of 1.97.

The MIS result was surprising.   Even Paul Roark's tweaking of the black-only Epson 1800 (?) to allow printing on glossy papers did noticeably better. (...and this did not use the MK/PK cartridge at all.)

Are my results surprising?  If so, does anyone know what might be going on? If not, is there any alternative to using Epson's pricey ink?  (Archival properties are important!)

Myron