In the real airplane, those gauges also show LEFT/RIGHT internal wing. It depends on what has been selected by the fuel selector knob. So, for this game, those totals are for the LEFT/RIGHT wing not external. For FYI, each external tank is 600gal or about 3,600lbs. So total fuel for the F-15C/D is about 24,300lbs. The F-15E is 32,000lbs.
Also, the only way an F-15C can jet it's weapons is if it jets the pylons.