I had the same problem today when I tried it. I know for instance with my shoulder muscles, I just don't have all that much strength yet. So I started at 10 pounds. At first it seemed like a 3, but by the time the set was over it felt more like a 7 rather than a 5. My weight machine will only increase by 10 pounds at a time so by the time I got to the third set I was doing 30 pounds and that felt like a 9 fourth set was a 10 (still at 30 pounds) and fifth set lowered to 20 was nearly impossible to do, the sixth set lowered to 8 and doing bent over raises felt like an 11, I couldn't even do it with proper form.

My shoulders are still really weak.

Some of my other excercises I felt like I started way too low like my back part. I started with a wide grip pulldown. I haven't done a lot of these, so I thought I would be weak on them and started at only 10 pounds. (it was only a 3) increased to 20 that was a 4, increased to 30 that was a 5, increased to 40 that was about a 7. At this point you are supposed to lower the weight, but I didn't, instead I took it to 50 pounds and hit about an 8. The reverse grip pulldown I can do at higher weight so it was at 80, but even that was only about a 9.

So I guess we just have to work at it until we find what level to start and finish at. The idea is that we should be almost unable to do some of them, (and that happened on my chest excercise, had to quit early).

I guess it just requires fine tuning. I wouldn't worry about keeping it at the same weight for some reps, it is still more intense because your muscles are more tired.