Thanks for posing your question. It is important to note that as Joe pointed out in his thesis, the actual as-built thermal resistANCE of SB wall assemblies will vary depending upon density and moisture content of the straw, the construction details employed and to a lesser degree, orientation of the bales.
Density provides one with an indication of the air content in a bale.
Extremely light bales will likely allow too much air movement within the insulation, and hence, heat transfer as well. Extremely dense bales will have much of the insulating dead-air pockets squeezed out of them so one would expect that such bales would have insulating properties similar to that of solid wood of the density and thickness. Water is a thermal conductor and as such,high moisture content bales will result in lowered effective thermal resistance. Thermal bridges may be inadvertantly have been created when detailing a SB wall. These will also reduce the effective R-value.