Most likely.

The box tends to sit slightly proud of the wall. When they plaster it in (particularly retroactively) they need to build up the plaster around the socket.

Thicker plaster means more insulation, and less condensation. Less condensation, less mold.

The ring around the socket also supports this. The edge where the new plaster gives way to the old. That point tends to be rougher, and so more prone to capturing mold.

It could also be thicker paint causing the same effect. Most people do the wall with rollers, but sockets with a brush, leading to a thicker coat.