Ever tried putting paper currency into a vending machine? It almost never works when it's flat. Give it a slight warp, a gentle curve and bingo, in it goes. Same idea. Because of heat, the stamping process, shipping, handling etc the case side isn't perfectly flat. There's always just a hair worth of play right in the center, if you push there it bows slightly. In order to be able to easily slide a case side into its mounting tabs, be it one that drops in from an inch away and has 30 tabs around the edge or just 2 that fit into slots at the front, there's always just a little play, the fitting isn't perfect or you'd have to have perfect lineup for the side to go on.
So what edge is loose? If it's the top edge, pull the side off and apply a small amount of pressure to warp the top edge slightly, very slightly. So when the side is returned, there's now pressure on the case side mounting tabs. Just like that bit of curve on the bill before sticking it in the vending machine. Only needs to be a couple of mm difference.