Her scaling is low because her cooldowns are pretty low. Landing Help; Pix all but ensures two Glitterlances once you have CDR, which is 520+1.0AP damage, with massive slows. And now throw in a Lichbane [0.75 AP] and her passive [0.15 AP] and suddenly this combo becomes:
Not knowing that Glitterlance can only land once tells me you obviously know nothing about the champion.
Glitterlance cooldown is low enough (40% CDR) to cast it twice within the pix duration. He knows more than you :)
Because people gauge the spell ratios of AP mids over a long duration of time as opposed to their burst in 2-3 seconds.