The Feed Rite sample I recently tested (pelleted beet pulp) also came back at 12.2% sugar. Since the starch has always been under 1% I quit testing that.
When I spoke to someone from Midwest Argi last year I was told that the molasses added type adds only about 5% more sugar to the mix.
So your 8% plain would be 13% with molasses. But if your plain tests at 13% you are now at 18%! Plain SHOULD range from 6-8% sugar - however I have tested it from 7-18%.