because in a rear surface mirror some of the light is absorbed by the two way journey through the substrate as well as being reflected backwards and forwards between the substrate's inside front layer and the mirror coating.
(some of the light; is absorbed; by the two way journey through the substrate; L:in a rear surface mirror) (the two way journey through the substrate; being reflected backwards; between the substrate's inside front layer and the mirror coating)
some of the light the two way journey through the substrate
is absorbed being reflected backwards
by the two way journey through the substrate L:in a rear surface mirror
between the substrate's inside front layer and the mirror coating