Fuel trucks

[Marcello]

"Less likely, I know, but anyway loaded fuel trucks don't just blow up like that AFAIK"

I know, even from personal experience, that generally fuel trucks don't just go BOOM Hollywood style as soon as someone sneeze at them. Unless you are hitting an half empty gasoline tanker or some such they will usually only burn, if that. US convoys would typically carry JP-8, which is relatively safe.
The reason for ammo class would be mainly the extra fire left after the explosion (to represent the larger amount of fuel burning) and the extra points value built in.
Anyway similar complaints could be made for the ammo trucks, I would expect game style explosions to be the exception.