PURELY CONJECTURE.....
The POH was formulated on the basis of a new aeroplane
It would NOT have taken account of today's pilots, who are not mechanically inclined or empathetic.
It would not have considered a carburettor ~40 years old, with worn float-needle and seat, worn throttle-butterfly spindle and Carb body, worn linkages..Maybe a bit of backlash in the Mag-drives, Plug-leads in less than A! condition....
Then there's the afore-mentioned Pilot Empathy.........Back in the day, they would have been enthusiasts , generally,- It was a rich man's hobby and the cost of a mechanic to keep everything spot-on was , overall, fairly insignificant.
The Aviator would know and really understand the operating -principles and demands of the petrol-engine....remember, Pre-war cars commonly had "mixture" "advance-retard" and "hand-throttle" controls for the driver to manipulate and keep his engine running efficiently.
Anyone who has read the Trevor Thoms series should be able to pass the exams....It's learning by rote! UNDERSTANDING is a different animal.
It's the understanding that enables someone to depart from the "one size fits all" POH GUIDELINES and operate their engine in an efficient and safe manner.
the "push a button" brigade will continue to have problems....An IC engine in reasonable condition (A defacto requirement of a C of A, i would think) will always start if it's given the right fuel-air mixture and the right ignition at the right time.......I'll admit that fixed mag-timing is set at optimum for cruise and therefore is not the ideal timing for starting -but that's what an impulse -coupling is for!
Know and understand your machinery apply logic to the symptoms and you will arrive at the correct solution
TMMorris is right!