Hard to know what goes on behind the scenes, could be one of many factors. In this video the Redback also sounds very, very different than it should:
To my ear the Redback is somewhat darker than many other Celestions, but not by a long margin. Some guys say it's really thick and wooly - not my experience.
I realise this sounds hypercritical as I've just posted comparison clips, but you really need to try stuff for yourself. There are far too many variables and inconsistencies in recorded clips.
Bear in mind the V30 used in our clip was the smoother variety (2002). A more recent one would contrast more with the Redback in the 5-6k region - in fact that's likely a factor in the Zilia video.