The flywheel horse, in my opinion, is much more accurate than a chassis dyno. A chassis dyno is like weighing yourself fully clothed, or nekkid, you can never be sure which way it might have been done.
Gear selection is an important factor on the chassis dyno, many other variables as well. The variables are reduced with an engine only dyno, but they still exist.
As stated previously, unless I see RWHP, I assume the number is flywheel