I think this has to do with the MD90s not having ovens. They get the same lunch/dinner service as the regional jets. I've encountered this a few times and I believe it has to do with the ovens so they cannot serve hot food as you would find on a 737/757/767 route with dinner service. I could be mistaken but I haven't flown many MD90s except on non-meal flights (Florida-ATL mainly) but I thought I heard they had the ovens stripped to add more seats and reconfigure the galleys.