Manhole covers are not flat. If you lift this one, it will have six reinforcing ribs cast into the underneath side, all triangular and vertical, and meeting in the centre. This strengthens it against vehicle weight.
The extra metal takes longer to cool down, so it doesn't freeze so fast where the ribs are . Also, air in the sewers will be warmer than the air in the street, and the additional area of the ribs absorbs this warmth and conducts some to the top surface.
Pretty, though. Maybe it was a design aim.
