Technical Side
Petrol contains a mix of octane (hydrocarbons with 8 carbon atoms) and heptane (7 carbon atoms). The numbers 92, 95 and 98 refer to the octane rating of the petrol, or, the amount of octane there is in the petrol. The higher the octane rating, the less likely knocking will occur, because octane burns slower since it has longer-chained hydrocarbon and is more resistant to igniting prematurely.
Sri Lankan Fact
I dont know you can remember few year back we had major issue with petrol and many vehicle got damage of it due to had quality on that time those who use 95 not effected so from that point onward people use and trust 95 than 92 .