We publish Stats19 collision data which is subject to recording guidelines set by the Department for Transport (DFT). The Police may publish figures that include NON Stats19 collisions.
A full list of non Stats19 collisions can be found in DFT guidance document Stats20. Essentially they are collisions that occurred on private land (mainly car parks), casualties that died from a medical episode as opposed to the collision itself and confirmed suicides. The DFT guidelines also state that if death occurred more than 30 days after the collision their injury is recorded as Serious.
Finally we usually publish data using calendars, running from January to December.