Ahh, so one of the other questions I asked was about the force policy and adherence to the ACPO Speed Enforcement Policy Guidelines 2011. Most forces, including my own have a central ticket office where any ticket below the prosecution threshold is automatically not taken forward. Not all forces have this set up though and don't always have this review. For example there are forces where officers can raise a postal requisition for speeding without the policy being applied. Whilst I agree data quality could be a reason for some or all of these stats, some smaller forces have policies where this could conceivably happen in real life (or at least they did in 2016). Like I said earlier, if they go guilty at the first opportunity, then perhaps the gaps in evidence wouldn't come to light.
It was quite interesting to see the wild differences in how forces nationally dealt with this, I expected it to be a lot more unified.