One huge complicating factor in the comparison. The trades on the internal exchange are actually real but volumes on centralized crypto exchanges are often little more than made up. That being said, the internal exchange has no fees and is very cheap in economic terms (accounting for the cost of locking up HP for RC) as you noted so at some point it might start to happen that a lot of wash trades happen, if anyone thinks it makes sense to inflate the reported volume. Clearly it doesn't now since almost no one pays attention to it.