This is a good thing in that it combines different characteristics into one conductor which allows it to be installed under a variety of conditions. For example, THHN by itself is not moisture resistant, but does provide a 90? rating. THWN provides a moisture resistants so now the conductor can be used in wet locations. And MTW appears to provide oil-resistance insulation the other two cannot provide. All the while the conductor insulation can be kept quite thin despite all its proerties.