Gragas doing a LOT of damage

I was just in a PBE game on Summoner's Rift playing as {{champion:58}} . The opposing team had a {{champion:79}} , and both teams were discussing in chat how the Gragas was ridiculous - on death recap, it showed Gragas doing 500 damage after killing a full-health (1.5k) Rengar. Whether it's just the tooltip misreporting or Gragas doing more damage than he's supposed to be able to, something wasn't right. Has anyone else experienced this with him?
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