That is a good point. It real life, each piece of bocage would be different- not some standard. For a game, there's got to be a bit of abstraction, but in real life they could find some good weak spots in tough places, yet in some there might not be anyplace that would be easier to get through. The combination of various plants of various ages, thickness, steepness of the banks, height, etc. would all vary to some degree.
In the game I figure that if they get a good average for the time it took to break through, that's good enough.