The first set you could probably get out a good 12 or so reps lets say, but stop at 5. So you're not really challenging your body at all for that first set. What exactly are you doing to your body for this first set if you stop prematurely at 5 when you can get out more reps? It would seem that you simply tire out your body, but you don't really "push it".
The second set you can probably muster out around 10 or so, but stop again at 5 so you are quitting the set before any real challenge to your body is imposed.
Wouldn't it be safe to say that really the only sets you are challenging your body is the 4th and 5th? It seems like the first 2-3 sets are sort of warm-ups, if at all since they are so few reps.