My Scrafty was hit by Hyper Voice for super effective damage. Hyper Voice on bulbapedia is listed as a normal move. Jellicent was then hit with the "normal" Hyper Voice and it should have been immune. It wasn't.
Then this happened. Jellicent used Ice Beam! It's super effective! The opposing Zapdos lost 36% of its health! Jellicent used Ice Beam! The opposing Zapdos lost 15% of its health!
Why was it super effective once and not the second time?