There has to be something wrong with Evelynn's E damage! Its way lower than it should and inconsiste

Evelynn E damage too low?
Uploaded by Senpai Moto on 2017-09-28.
Maybe Im just very stupid and missing something. But if I have 972 AP, my E deals 140 plus 6% (+34%) of max health. Against a dummy with 10,000 HP and 100 MR (50% damage reduction) that should be 2070 damage. But it deals 307 damage. (00:15) Against the red buff with 3360 HP and 10 MR (~10% damage reduction) it should deal ~1300 damage but its 527 damage. (00:52) Apart from that the damage seems to be inconsistent? Bought some Magic Pen against the Dummy and one time with W -> Q -> E, my E dealt 566 (02:22), the trys right after dealt 526 damage. (before and after 02:22) In the end I just spammed non empowered Es and dealt 425, 400 and 440 damage (02:56) (Im not sure if this can be the result of one of the new runes but I guess the things above shouldnt) Video for reference (very sry that I didnt have time to cut it, just check the timestamps)
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