It generally takes a few minutes before your latency/ping reading smoothes out. This is true with or without WTFast. Often right after you get into the game, your ping will read really high, or really low (often 0ms). Please wait a few minutes and your ping rating should settle to what the true latency really is. In most cases, you should notice a significant improvement in your ping rating while you run WTFast.
The latency reading also only tells you part of the picture. WTFast makes your game connection faster (lower average latency), smoother (less ping deviation) and stronger (less packet loss, and fewer hops to your game server). The reporting will give you a sense of the various ways WTFast optimizes your connection.