Coverage maps

I've noticed that in some countries cell companies use dual coverage
maps: one that is valid only outside of buildings, the other that is
also valid inside. The maps are also separated by different signals,
including 2G, 3G and higher. Has anyone seen such coverage maps in the
US? I wonder how they can achive such significant radio penetration that
makes it worthwhile to show a separate coverage for inside and outseide.
Could it be because they use different frequency where they intend to
have good signal inside of buildings, such as in large cities? I know
that penetration is dependent on frequency used, but I'm not sure if
this is the key to such dual coverage maps.