The reason behind the need to assign the Trek Bible readings separately is that they can be completed on either a 2-year plan or a 3-year plan. The Bible readings are not tied to a specific Trek Book. If you look in the back of each Trek book, you will see the options. Some clubs have a 2-year Trek program and some have a 3-year program.
In addition, students who start Trek "late", have the option to catch up on Bible readings at their own pace is they are Citation bound.