Here's the deal...
When you have a sudden cardiac arrest, your heart has already progressed from a perfusing rhythm to a non-perfusing rhythm.
i.e., your heart is not pumping blood to your brain, and you are clinically dead.
The tracking systems report the "initial rhythm" as whatever the rhythm was when the first person with an AED or Cardiac Monitor got their gear wired to the patient.
If the rhythm was "shockable" and the device was an AED, we know that it was one of two or more well-known rhythms.
If the rhythm was described in terms of the specific rhythm, the device used was a cardiac monitor.
The officially recorded presenting rhythm is reported as "shockable" if an AED fired a blast and as whatever the paramedic on scene saw if the EMTs were the first on the scene and attached a heart monitor.
Here's where the problem comes in: If the EMT's / Paramedics took a little while to get there, the initial presenting rhythm might have been VF at the time of the arrest, but a lot different at the time the EMT's arrived.