This(or something very close to it) was discussed before here. Roughly speaking, Incantation of Lightning "wastes" a few magnitudes -- but this "stretching" seems necessary to simultaneously achieve all of the following:
a) maintain coherent and concise Auram guidelines
b) keep Incantation of Lightning at the same level as it was in previous editions
c) keep the damage coherent with that produced by a "realistic" lightning bolt
d) keep the level necessary to deal that damage with Auram at least as high as the level necessary with Ignem -- the "gold standard" for damaging spells.