Why are there two different base covers?

The main difference lies in the materials used. The standard base cover is made from shapely cast aluminium, powder-coated in accordance with DB 703 fine-textured metallic iron, and is simply placed over the base.
The sprung cover consists of a zinc-plated steel cap with stainless-steel springs underneath. The cover’s clamping strength on the base can be adjusted by manually compressing the stainless-steel springs. The springs, however, make it sit firmly on the base, and can only be removed using a levering tool.