Look again.. You are discussing 2 different actions here.
The Percentage Activity Table lists Riding as requiring 15% activity.
The Percentage Activity Table lists Mounted Combat as requiring 100% activity.
Mounted Combat is listed below Riding. This means that they are considered to be two separate actions.
The section on Mounted Combat discusses how much of your activity percentage, during Mounted Combat, is devoted to controlling the mount (100-riding skill = modifier to OB). Actually, you are doing a specific type of simultaneous action, riding and fighting, and it is your skill in riding that determines how much of your OB you have available.
So, both percentages are absolutely correct. They are for different situations.
The percentage for Riding is for making Riding maneuvers (i.e. making the mount jump an obstacle, controlling a frightened mount, etc.) while in combat/tactical time.
Mounted Combat takes 100% activity, and your available OB is determined by your riding bonus. The description of Mounted Combat could likely be better phrased as it talks as if you are talking about 2 different actions here when you are really performing a specific type of simultaneous action.
