It could be something to do with the vertices of the correctly lit bridge overlapping the vertices of the dark bridge.
no, because the VDC says what I said..
see??
VDC said:-StaticPropLighting
Generate per-vertex prop_static lighting. This is always enabled for light_spot entities.
Noteepending on the number of static props in the level, this can dramatically increase the size of the .bsp file and the length of time needed for vrad to finish.
Note:When using this setting, it disables all info_lighting entities in your level and it will light models as if there wasn't one linked to them.
whilst the poorly lit one is inside one of those rock props. However its compiled with -staticproplighting so it shouldnt be using the origin at all for lighting.