Also, you have to look at it this way: Hits are not apportioned so that you do just enough to a guy to kill him, then move on to the next guy. If you hit one guy with 5 lances, five guys aren't going to drop dead just because one guy took enough damage to kill five men.
That Arrakis Heavy Cavalry unit had 63 men in it. The SADA's Brewmasters unit they hit had 61. I don't think it's unreasonable to assume a 1-1 targeting from a cav charge to a line of soldiers. The SADA's Brewmasters were completely wiped out by that charge, and the cav unit only took 10 casualties.
Cav units tend to be smaller than infantry units due to expense and the fact that it's hard to be able to command a huge cavalry unit. That means they are going to tend to only damage one or two units at a time, unless their enemy has a lot of small units.
