"I'll try to get more in-depth about the roads that are not on the MVUM."
Through my volunteer work in the Forest Service Adopt-a-Road program, I have had a lot of things explained to me:
You can't take just one map. Topo maps don't show you the MVUM, the MVUM shows only roads designated to motorized travel, and property lines for the National Forest. On the MVUM, shaded areas are National Forest, un-shaded areas are not. If the property owner gives the Forest Service an easement across their land, you will see a F.S. designated road marked on un-shaded areas of the MVUM. If the Forest Service does not have an easement, the designation for motorized travel ends at that property line. The road that continues from the property line is designated as "user created" and may, or may not have signs on the ground to restrict traffic.
Someone already mentioned the Travel Management Rule, announced in 2005. Many of these roads predate that rule by 140 years, they don't cease to exist because someone waved a pencil in the air. Some of these roads may service things like cell towers, radio repeaters, or weather stations. Access may be restricted to service vehicles. Some of these roads may not be clearly marked as the end of the MVUM, or markers may have been vandalized by that 1% who have no concern for their actions. Just to keep things confusing, there are still signs pointing to areas that have since been closed.
I am glad your area is well marked. The primary system that I volunteer on will get new signs this year, a lot of new signs.