The fact that you don't find it hard to understand is probably an indication of the quality of the material. I'm a professional courseware developer (though not for aviation) and I know it's pretty hard to explain complex technical subjects like N/S compass errors accurately, properly and understandably.
If they are able to do so in a way that you understand the underlying fundamental problems, how these problems were solved and what the resulting effects of the final solutions are, to the point where you only need to learn a simple mnemonic, I'd say they've done a pretty good job.
And it means you've got a choice. You can either remember the underlying fundamental principles/solutions and use those to arrive at the proper solution in the written exam, or you can remember the mnemonic. If the FAA written exam is any good, it shouldn't matter which approach you choose. Although I might say that in a lot of cases, if you remember the mnemonic you'll be able to find the correct answer a lot quicker.
But at the end of the day you've got to remember that a compass has errors when turning and accelerating, and how these errors they affect your compass reading.