Yes, you never get 100% mixing in the time available. Most oil and gas burners work on around 10% excess air to get complete combustion. BR quote 20% excess air in the Black Book but a loco firebox does not have the benefit of fuel/air pre-mixing or much else to aid complete combustion. From the moment the air hits the firebed, it only has a very short time for the oxygen moleclues to mix with the carbon and hydrogen before the gases reach the tubes where combustion effectively ceases. That is one of the reasons for having a brick arch. It lengthens the path the gases take and gives longer for combustion to take place. The combustion chamber provided on some locos similarly adds to the length of available flame path.