If primer blast did not move bullet and if charge ignited practically instantly, it would be very difficult to resolve the following results (and many others). First, it is quite common for an increase in neck tension in 44 Magnum loads to result in significantly higher chamber pressure; conversely, substitution of a primer that is otherwise demonstrated to be "hotter" will, in the same load, generate significantly less pressure.
One could conjure all manner of explanations for both of these results but only one follows the tenets of "Occum's Razor" (providing a simple explanation, rather than relying upon ever more convoluted conjectures). That simple explanation is: greater neck tension tends to reduce bullet movement in response to primer blast, which occurs before effective propellant ignition begins – it results in a smaller boiler room; conversely, the more violent blast from the "hotter" CCI-350 moves the bullet farther before effective propellant ignition begins – it creates a larger boiler room. If propellant granules ignited essentially instantaneously, these results would be mutually exclusive.