It doesn't take much force to mess up sheet metal. Hoods, for example, are wont to warp in most frontal collisions. Doors cave in effortlessly when nudged in the side by another car. And it doesn't take much to sheer off the entire top of a car when it underrides a semitrailer. A witness may see only the damage itself and then speculate that the car must have been speeding.
Witnesses often change their stories after they've had time to reflect back on the event.
They may make one assertion to police at the scene, then revise their statement after talking with family, friends, fellow workers, attorneys, or insurance agents. Unscrupulous interviewers, usually an advocate for one side or another, can easily sway witnesses by the way they ask the questions. Then there's that time-lapse memory thing. A lawyer calls in a witness two years after an accident. The witness hadn't even thought about the accident until they received a subpoena.