While every guild or school of magic will ascribe different a name to the theory, all mages inevitably stumble upon an inescapable fact - only the Archons can will magic into being, everyone else can only cast magic by invoking the sigil of an Archon that has come before. None know how the Archons do their magic (it seems to be different with each Archon) but theory suggests that once an Archon starts to invoke their powers, anyone else who comes after that Archon could (with the proper knowledge) tap into that power with the proper sigil and know-how.
As a result, magicians merely imitate. They take the Archons' sigils, study them for secrets, tweak and twist them, experiment with them, and commit the various forms they can take to memory. Through rote practice, they learn to perform specific and discrete effects. Archons, however, have neither such recourse nor similar limitation. Their power comes from within, not some external template. Each Archon manifests their own peculiar talents, sometimes abilities never before seen, sometimes twists on powers that have been seen in the past. Further, those abilities can change over time, becoming more potent and flexible.
This observed law has given rise to a common question from young students: could one guess at a sigil of an Archon not yet born (or not remembered by history, or simply unknown to the caster) and successfully cast a spell? Experienced mystics would tell you this is impossible, for they know that it's not enough to just mimic the gestures of a sigil - one must have willful, conscious knowledge of the Archon and the intended power being channeled.