Flash point ans self ignition temperature

Auto ignition temperature depends on the chemical structure of the component and is depending on carbon chain breaking temperature. Hence petrol which is having some complex structure than diesel will require more temperature for chain breaking.
Ignition will occur after chain breaking.Hence auto ignition temperature is higher than diesel.

But in case of fire point and flash point it depends on density of the fuel. Diesel has high density when compared with petrol. High density fuels will have higher flash and fire points. This why diesel has high flash point than petrol.

Flash point and auto ignition point are completely two different things. Flash point is related to volatility of fuel while auto ignition point is related to the activation energy needed by the fuel for combustion. So, there is no point in comparing two things which have no relation.