[narwan]

The difference in number of shots is based on the assumption that the tanks have either started with some supression or get some suppression during the turn. The 3 tanks plus commander (4 total) will have more rallying attempts so a better chance than the 5 tanks of actually getting the 3 shots each. The example is just a theoritical case where three unsuppressed tanks (supp<4) actually have a better hit combined hit chance than 5 tanks with low suppression each (between 4 and 10 approx). For main guns around the 75mm range for medium exp crews (the most common actually) that means the ROF for the main gun will drop from 3 to 2. Especially if moved first for one or two hexes. The tanks with suppr less than 4 will still have 3 shots each. Which makes a big difference.

The math is simple, the 1/3 refers to the first shot (at 1/3 of the max hit chance), the 2/3 refers to the second shot (at 2/3 of the max hit chance) and the 3/3 refers to the third shot (at the full max hit chance). The add the factor of the max hit chance for each round fire by the 5 tank platoon and do the same for the 3 tank (plus leader) platoon and you get a number which is an aggregate for the total hit percentages for the formation. That way you can compare formations of different size.
In this theorerical example the command tank of the 4 tank platoon does not fire at all but is assumed it's rally ability will get each of the three subordinate tanks to fire 3 times where the lack of additional rally attempts for the five assumes they'll be stuck at 2 shots each.

The previous example illustrates that even in absolute chance the 4 tanks have a better hit chance than the 5 tank platoon. But even if 2 out of the five tanks in the 5 tank platoon have three instead of two shots the 4 tank platoon (where only 3 tanks fire at all!) will still be more effective on a per tank basis. I'll save that math for later.

But in short, keeping commanders back to rally their subordinates tend to lead to more effective formations. The real trick is to find the right of ratio of commanders vs subordinates.

Narwan