>>527091585
In both cases either oversight or because they aren't Sabers. It isn't abnormal for those classes to have Riding, hence oversight, but Christmas made a big deal over Bradamanta lacking her Pegasus (which Astolfo accidentally stole) so it could be as a Lancer she doesn't get the skill but as Rider she would.
It's weird cause in Tristan's profile it notes he HAS Saber qualifications so it isn't far-off for him to have Riding.