Why do fence contractors charge extra labor cost for hitting rocks or roots?

Soil conditions are different in different regions.  Here in North Texas we have several soil types.  Southlake generally has very good digging where as Saginaw can have some of the hardest rock just below the black gumbo.  Nonetheless, when we can use our auger to dig a nice 30 -36″ holes in about 5-10 minutes, life is great.  However, when we have to break out the rock bar or jack hammer, that hole will now take 30 minutes to an hour dig it.  Well, there is the difference.  That extra labor can add 1-2 days to job depending on how big the job is.  We generally charge $30 extra per hole when this happens.  The extra charge just barely covers to actual labor and really cuts into profits.  We all know that extra labor has extra costs.