What is the meaning of the pencil edge mirror?

There is a slight <b>curve</b> on the pencil polished glass edges. On Jun 27th, 2017:

Question

There is a slight curve on the pencil polished glass edges. On Jun 27th, 2017:

Hagrid P Hazard · Accepted Answer

There is a slight curve on the pencil polished glass edges. On Jun 27th, 2017: