I don´t know why, but I think that may be part of the mapping specs? Information was in the PM at least.
"In the terrain there is a telephone line which is easy to mix up with a power line as it is not on the map. This
affects all legs."
I'm sure there was a valid reason! Actually i had read somewhere about the powerlines, but because this one was close and almost parallel to the mapped one that i had used yesterday i didn't think in the moment that it might be another one that wasnt marked.
I'm generally all for having terrain information drawn on the map and not to read in the PM but i don't actually know the rule or reasoning and of course probably wouldnt be so bothered if i had managed that leg a bit better :-D