There is nothing suspicious in the lack of NR symbol, baboonery is correct, this is a convention amongst most TfL maps. Think about if you wanted to travel from Crystal Palace to London Bridge (and ignore the existing half-hourly Southern loop service). It makes most sense to change from the ELL to Southern at Sydenham (higher chance of a seat) or at NXG (nearer to destination and also higher chance of a seat as some people change for the ELL). Whilst you can change at any of the other stations, nothing is gained, and its probably more efficient in terms of timetabling to try to encourage everyone to change at two places rather than ad-hoc at all stations.

Similar things happen all over the place. For example, on the equivalent Picadilly line maps for the Tube, interchange to the District line isn't shown for Gloucester Road even though you can, simply because nothing is gained and waiting for Earls Court/South Kensington works just as well.