It's the only possibility for the system to have a logic and that's also explain the unit info bug. Actually they didn't use 2 different formulas, simply put who wrote the unit info code made an addition of two different variables which weren't supposed to be added (and aren't added anywhere else in the combat code).
Or they didn't update the code that linked the unit info to the new armour upgrade system, so it still uses the RTW code that shows +1 for every armour upgrade.