Following are the reasons:

1. Bunker oil is refined oil and not crude oil.
2. Bunker fuel is required to have a Flash point temperature of 60 deg C and above as per the Classification Society rules.
3. Bunker fuel is not flammable under normal temperature and pressure, as carried in the bunker tanks of ships. Provided the bunker tanks are not heated above around 40C, they are not considered flammable
4. Further, owing to the smaller size of the bunker tanks, the atmosphere usually is "too rich" to cause combustion