If I plot a route from Hyde and Bay to 6th and Mission, it routes me over Nob Hill. If I change it two streets to 4th and Mission, it routes me around the Embar

If I plot a route from Hyde and Bay to 6th and Mission, it routes me over Nob Hill. If I change it two streets to 4th and Mission, it routes me around the Embarcadero, which is much flatter. Is there a reason it takes the more direct (less flat) route in the first instance? Having played around, the route given seems to vary slightly as I move an endpoint slightly at either end and then massively recalculate - perhaps it caches certain segments and favours those? Just curious really, it's a cool idea!