In this part of Chapter 6, the narrator talks about ways in which new recruits get killed because they do not know what they are doing. He mentions one reason right after the quote you provide us, and he provides further reasons later in the chapter.
The first reason is because the new recruits do not know when it is safe to take their gas masks off. They don't realize the gas settles into low places and that people in low places still need their masks after people higher up are safe.
Other reasons are implied a few pages later. The narrator is talking there about all the things the new recruits need to be taught. We can infer that they die more than the old hands because they don't know you can run from mortars, because they don't know how to tell the sound of the small shells that are more dangerous, because they don't know how to act when their position is overrun by the enemy.
All of this lack of knowledge explains why so many more new recruits die when compared to the old hands.