Now suppose you perform a Take-Off on a runway without clearway eg close-in obstacles. Now using Wet compared to Dry leaves you with less height in the same spot (15ft compared to 35ft) however regulations demand that you cross any obstacle by at least 35ft and make no difference between a Wet and a Dry Runway. That means, that when using Wet Data you still have to achieve 35ft over the first obstacle and thus your climb gradient must be better. This fact is being accounted for in the performance calculation and results in more excess thrust when compared to Dry Runway Data.
This is where you're a bit off. The regulations demand that the
net takeoff flight path clear obstacles by 35 feet. The takeoff flight path and net takeoff flight path begin at a point 35 feet above the takeoff surface at the end of the takeoff distance, regardless of whether the takeoff was conducted under the dry or wet rules. When the runway is dry, the airplane must be at a height of 35 feet at the end of the takeoff distance. But, when the runway is wet, the airplane need only be 15 feet above the takeoff surface at the end of the takeoff distance. Therefore, although the net flight path obstacle clearance is the same for both wet and dry takeoffs, for the wet case, the actual clearance will be 20 feet less.
The way this was done in the rules was to state in FAR/EASA CS 25.115(a), "the takeoff flight path
shall be considered to begin 35 feet above the takeoff surface at the end of the takeoff distance..." That way, there was no need to change the operating rules to accomodate a 15-foot net flight path obstacle clearance for the wet case, and no need to build a new set of 15-foot obstacle clearance charts.