UC Davis don't have a test that shows what dominant agouti is at the locus. Instead, they have a test that looks purely for the recessive agouti - the "a" as opposed to the "A, At, A+". When they run the test, the results tell them how many copies of the recessive are there, so results look like this: a a, a _, _ _. They then fill in the blanks, but don't actually have any idea what dominant allele is present - only that it isn't the recessive one.
I still think the filly is brown roan.