Either the Gistl attribution is false, and just assumed widely because of the similarity in technique, or both glassworks have produced similar looking objects.
I also checked to be sure whether the factories were part of the same group, but the Christinenhütte is not the same as the Gistl hütte. Gistl eventually became part of the Riedel-Nachtmann group, while Zwiesel was long part of the Schott Zwiesel, but apparently had a management buy-out in 2001 and now is just Zwiesel again (but they also use the name Jenaer Glas).
Astrid