This is actually due to the horrendous change in 4e lore, that they carried forward into 5e. Tieflings as we knew them in 2e and 3/3.5, have been replaced with this...
To be fair to 5th, they acknowledge the 4e canon for what happened (Asmodeus' gambit), but they've also negated it by saying that that one time transformation was just that - a one-time deal. Tieflings born since have begun to show the same variance they always used to have, it's just that now, as a result, Asmodeus teiflings are still the most common variety.
Regarding the ones here in the grove (this is more to Gimbal whom I assume is less familiar), there's a background lore reason for a large group here right now, which coincides with the unusual number of them. One of the more recent 5e game books was Descent into Avernus, which involved, as a result of other things, a great many spontaneous teiflings and a large amount of anti-tiefling sentiment arising more strongly in the city of Elturel and its surrounds. This group come from there, where they have all unilaterally been made unwelcome.
