My guess is you've made tests using direct fire, no?
Made an experiment with clones of the same M113GA1/Tampella 120mm with RoF 3, 5, 7 and 9, purchased in groups of 6 each
RoF 3: 3 rounds fire mission fired
RoF 5: 6 rounds fire mission fired
RoF 7: 8 rounds fired
RoF 9: 10 rounds fired
The game deals differently with direct and indirect fire re RoF, in DF mode all the other factors are important, in indirect mode the RoF is dominant as it seems.