River Crossing rule rationale?

[Matt W]

High numbers indicate a river with low banks and easy access to the banks, also a slow current and a lack of mid-stream obstacles. Low numbers mean the opposite.

Of course, anything that creates a difficulty to the engineers can cause the crossing number to go down. As a result a designer might look at a situation where the crossing appears easy but the force attempting to cross was inept and simply reflect that lack of skill by lowering the crossing number. Not all engineers are equally capable. Note also that in some cases the designer may reflect the defenders resistance in these numbers as well as, for example, the case where defenders sharpshooters are particularly effective in picking off the engineers.

So the river crossing number is actually an amalgam of factors, some specific to the river, some specific to the attacker and some specific to the defender.