There are different versions of a particular geometric shape that can be a K3, K4 or K5. K5's are in Masters only. There can be anywhere from 2 to 3 in a Masters sequence. It has usually been only two with the traditional long sequence we have been using till now. Also, not all K4's are really equal to each other in difficulty. A K4 Square Loop on Corner is definitely more difficult to do than a K4 Square Loop with the exact same roll elements.

As you progress up the classes, you start to get higher K maneuvers introduced. There are plenty of K4's that can challenge an Advanced pilot and prepare him for Masters. A simple example, a K4 Triangle Loop in Advanced should adequately prepare an Advanced pattern pilot to fly a K5 Triangle loop in Masters. Difference being one or more elements added.

The progression of the classes is set up to introduce new skill sets and difficulty at each level.

When Derek posts the final approved sequence development guide on the NSRCA website, you will see the established boundaries and considerations taken based on past sequences and past sequence committees concensus described in the Annex section. They are intended to be reevaluated each time new sequences are developed and can be adjusted depending on the state of pattern, but can only be changed if it can be justified. This is done with the intention of trying to limit the creap in sequence difficulty over time. Right now Masters has a hard ceiling established on maneuver difficulty. It does not allow complex integrated roll maneuvers, certain snap and roll combinations, and rolling circles. To me there is a greater jump between Masters and F3A then there are in the other classes. I can't even really do a rolling circle. I hope this never changes, otherwise hastalavista for me.