Either a ) you have a sld wired in somewhere, b ) the speed limiter can sometimes be a bit slow to notice you have gone past the limit, usually the case when you do a top speed pull on a dyno, due to light load on rollers or just where there doing a slow pull, or, c ) you didnt actually reach the speed limiter, things can be a little off, maybe you have an aftermarket ecu with the emanage wired in, such as a jam with the emb piggybacked into it.