What is so hard?

I can't quite figure out why Jillian's DVD seems so hard. The basic moves of the Level 1 portion are not hard. It must be the combination of the moves and the pace of the work that make it hard.

Jillian's DVD contains three 20 minute workouts: Level 1, Level 2, and Level 3. Each level is broken down into three circuits of strength, cardio, and ab work that are repeated three times each.

In Level 1, representative exercises for the strength training include: push ups, squat presses, lunges with bicep curls, and the chest fly. The cardio work includes such moves as jumping jacks, jump rope, and butt kicks. Traditional crunches and bicycle crunches, among other moves, are in the ab portion.

Again, individually, none of these moves are all that difficult, and most are actually easier than those found in Jari's DVDs, so I'm not quite sure why I have found Level 1 to be such a challenge thus far.