Normal
You can't plan for zero clearance. That's ... well, at the very least, bold. Manufacturing tolerances are not zero. Insulation and wire thicknesses vary. Wires bend, and not perfectly either - and so on. So going with no clearance at all would in all likelihood cause bulging side panels, trouble attaching them, pinched wires, etc. And nobody wants their $80 PCIe 4.0 riser cable to be slowly cut in half by the motherboard tray squeezing it against the side panel, nor the side panel on their premium case not attaching properly.