I'm definitely not an expert on how routing engines make determinations so it's hard for me to say precisely. Would be an interesting question for folks in say the OSM US Slack or the global Discord server. 

Most algorithms work my "weighting" with different roads having a higher "cost" to route through. So a route marked with high max_speed or surface=paved  etc will have a lower "cost". 

I suspect that if the surface there is unpaved or it's marked as max_speed=15 (which is typical for unpaved roadway) or if it's actually a single lane... that the routers will try to keep folks from being directed through there.
