I think the normal AP cost of movement on flat grass is 2. Thus, Pathfinder (which reduces the cost to a minimum of two) would not normally have any effect on this; it simply remains at two. Add an injury that increases the cost by one, and it takes 3 AP to move on flat grass. I guess the injury effect is applied atop the Pathfinder one, which seems to make sense. But I expect you’ll find that the character now (with Pathfinder) requires 1 AP fewer to move in dense forest/snow or in swamp.