Of course: As units grow in strength due to bonuses (chevrons, upgrades), the relative difference between weaker and stronger units is diminished. Thus, 3-gold chevron units are overall far more equipollent than newly built ones.

There is a restriction to your calculation, though. If you take whole units and determine their overall attack strength, even if taking into account equal morale/general etc you were right if the whole unit would fight at once. Since they very seldomly do so, overall "weaker" units with better individual stats stand a far better chance of deciding a combat in their favour. An extreme example would be a battle on the wall where the match-up arcani vs. warband might well result in a victory of the former.