There are a few reasons why the hp varies from boat to boat. Firstly, if the boat has been built by an owner who will predominatly use it on rivers, there is no reason for the higher hp because there will never be the need to get up on the plane. In this case any excessive horse power would only be a waste of money at the build stage, and throughout its life the boat with a higher hp would not be running at the most efficient speed for the engines, thereby causing more wear and tear and shortening its life.
Secondly, even if it was intended for offshore use, the first owner might not need the maximum speed that the boat might be capable, but instead might wish to reduce his fuel bills while still cruising at a reasonable speed.
It is always worth trying to speak to the selling owner about this and any other points which might concern you, but otherwise ask the broker who might be able to help you.