Originally Posted by
Jim Dull
The walls appear to be some kind of plaster coating over drywall.
You are, I presume, referring to interior walls (because you said "drywall"), could be a one coat veneer plaster coating over special gypsum board (drywall). Usually, the gypsum board used for plaster is 'blue board', which has a 'bluish color finish', hence its name. The special paper on blue board allows the plaster to adhere better and provides for better and easier application of the plaster.
The "plaster" setting, I would think, would be for 'real' plaster, three coat stuff. It is possible that the veneer one coat plaster is dense enough to change the readings when set to "drywall".