The unusual maximum speed readout that you are incurring is due to the cordless computer unit registering interference from an outside electrical field. Sources of such fields include RR crossings, power lines, security systems, radio towers, home computers, televisions, radios, cell phones, garage door openers, high intensity discharge lighting systems, etc.
When all speed related functions are accurate except maximum speed, this is almost certainly the case, as cordless/wireless models manifest such reception of outside interference as an abnormal max speed readout as described.
Unfortunately, there is no solution for this, as the FCC requires electronic devices such as bike computers to accept all such outside interference.