I agree that figuring out the modifiers for a bell curve system is less intuitive than for constant systems. But I've often wondered if it's even worth it to have a point-based system.
Now, this may be heresy for me to say for a wargame, but I've never completely understood the concept of "points armies". Jon Tuffley in his Stargrunt II game didn't provide any points system for an "effectiveness" calculus. His reasoning (which I find difficult to argue with) for this omission follows three main points:
1) Firstly, while even though it may be possible to gauge the effectiveness of a single unit, when you look at the group as a whole, the groups effectiveness is more than the sum of its parts. In other words, you have to look at the group holistically.
2) Secondly, the effectiveness of units is extremely context sensitive. For example, if you design a bunch of units that have really powerful long range weapons, that's not going to do them a load of good in heavily forrested areas.
3) Finally, and this is where I think the "heresy" part comes in, having a point total destroys the mission based atmosphere of the game. Instead of creating imaginative campaigns in which the odds are never even, a point system encourages a competitive mindset in which you simply throw armies at each other.
Unfortunately, most people are loathe to play unbalanced games. And yet many battles are unbalanced, precisely because you don't want to fight your enemy when he's as strong as you are. You maneuver and force him to split off his forces and fight when and where you choose to fight. Point totals encourage a type of gaming where these imbalances do not exist, and hence there is less creativity and imagination involved in setting up the scenarios.
But getting back to the statistical problem, I suppose you're right that considering the meta-play, there does turn out to be conditional probabilities which will in effect make the odds more bell curve like. Still though, I think the interplay of having a more diverse variance and deviation in the probability distribution would make not only for more interesting gameplay, it'll change the way you have to think about your tactics and strategy. Granted, it will be less intuitive, but I think having modifers which are also skewed to the probability distribution and density functions which follow bell curves would be interesting.