As Mic said, there is a lot of leeway in how you can lift with P90X.
I found that I could lift heavier with P90X than I've ever been able to lift with Cathe. I've done 35# one-arm rows with P90X, but I use 30# or even 25# for Gym Styles. I was able to do 15# 1-arm tricep extensions and 20# dumbbell curls (for a set!) with P90X, but I can't get that high with Cathe.
Aside from the "work at your own pace" style of P90X, I think there are two factors that make this possible. One factor is the work-to-recovery ratio. In P90X, you always take the time to write down what weight you used and how many reps you did after each set. There are also recovery periods for dynamic stretching after several sets of exercises.
The second factor is the arrangement of exercises, which also allows for recovery. When you do P90X chest/shoulders/triceps, instead of doing all the chest work before going on to shoulders, etc., you do one chest exercise, then one shoulder exercise, then one triceps, then repeat the sequence, then take a recovery period. Same with chest/back, back/biceps, shoulders/arms.
I love the whole P90X philosophy of working hard, and recovering and restoring (with in-workout stretches and rests, with yoga, with recovery weeks).
I agree with Mic that P90X is great for upper body, but, although I found the leg exercises challenging at times, and they firmed my quads, they didn't keep my butt where and how I like it to be! A perfect combo would be Cathe for lower body and P90X for upper body.