1. The other is a Nexus. A similarly spec'd tablet like the Note 8 for example would cost almost the same as your HTC DNA. Nexus tablets are always cheaper.
2. Manufacturers like HTC, Samsung, etc, put premium on their devices because well, they charge an overhead for the features they put in the phone (TouchWiz and Sense for example), and the costs they put in doing the research for making the updates (salary for the workers, profit, etc). Nexus makers can get the profit they want from the hardware since they won't be doing any of the software work, Google makes all of that.