Reduced Damage to Inhibitors?

I played a custom Rift game after updating to 5.2 with Arclight Vel'koz (awesome skin, BTW!) and just wondering why hitting a turret with 100 Armor & MR did over 200 dmg, but hitting an Inhibitor with zero Armor & MR did barely over 100 dmg. Is this a bug with the skin, map, custom games, or is this a new challenge thing (you need an AD to attack the inhibitor or you'll be standing there a while)?

Update: Seems like maybe only AP champs are doing reduced dmg to inhibitors and the nexus. Did a custom SR game with Battlecast Vel'koz, then a game with classic Tristana, then a game with Justicar Syndra. Tristana was the only one able to significantly damage the inhibitor (200+ per hit). Both Vel'koz and Syndra were doing 100+ dmg to the inhibitor turret, but between 85-115 dmg to the inhibitor.