Do you live in a place where when the wind blows, it is a constant speed with no variation?
I know the wind indicator in the game client shows one speed, but I'd bet that the variances you are seeing are a result of the software randomly varying the wind speed at the time of the shot.
Even on a real course, if you're playing in to a 18 mph headwind, you can expect it to die down and pick up at any given time. If your shot is high enough, you can also expect that the wind up there will be different than what you are feeling on the ground. The wind will also be different where the ball lands compared to where you are standing.
The only problem I see here is that the wind speed indicator never waivers. It would be nice if WGT built in the wind speed variance in real time and showed it on the indicator instead of varying the wind behind the scenes at the time of the shot. Even then, you still wouldn't know exactly what the wind was doing 50-100 ft in the air or 150 yds ahead of you.