Not only does vacuum forming increase the density it also decreases the size of the pores.
This is the key. Pore size is likely the second most important factor (behind density) with respect to conductivity, AOTBE.
As Tom noted, this is a function of the manufacturing process explaining why process matters. It also explains why the WG LD buff firebrick has such low conductivity.
1) Since conductivity is directly correlated to density, it stands to reason that AOTBE, the material with the larger pore size will be more conductive because it will therefore have less pores and areas of higher density. This is confirmed by the model in this paper: http://web.ornl.gov/sci/buildings/2012/1992%20B5/013.pdf
2) At high temps, radiant heat transfer will also be a factor. Heat moves through a porous material by thermal IR in addition to conduction. The higher them temp, the more IR is a factor (a fourth order relationship). When it comes to IR, I suspect the smaller the pore size, the less efficient the heat transfer as the thermal energy must be emitted, absorbed, and readmitted more and more times as the pore size gets smaller.
So, the efficiency of both conduction and radiant transfer decrease with pore size, thus smaller pore size = lower thermal conductivity, AOTBE.