Why not?
Because the mutational changes thru time recorded in specific sites in the genomes of two related emergant species are in different locations in the genome's DNA slice. We can usually tell what the extinct common ancestor's genes looked like, by cancelling out both sets of changes we observe in their disparate offspring. Since the changes are infrequent compared to the length of the genome itself, that doesn't leave crawlspace for an argument that the supposed ancestor was really a forerunner--what with being extinct and all.
"We can usually tell"
But not always? So it's open to interpretation?