All the vehicle data used by insurance companies is supplied by the DVLA who request it from motor manufactures/sellers who import/register the car when new; so if your reg no doesn’t match your car details there may be a few reasons;
a) Wrong number plate fitted when new
b) It’s a stolen car and someone has cloned a legitimate reg number
Its more than likely a database error but I bet if you had an accident the insurance company would look not to payout citing the car/engine details and reg don’t match, head straight for a main dealer to confirm VIN/Reg/engine block/car details are correct and then inform the DVLA.
Make sure the insurance company you sign up with know there is a problem with their database, in my experience when it comes to car insurance it really pays to cover your ar$e