Hi,
The fluctuations you see in your test can be attributed to either the number of silly mistakes you may have made or to some question types(read topics) appearing that you aren't too well versed/confident with. Anyhow the kap test diagnostic tool is very powerful, you may like to go back to the test diagnosis and examine the mistakes. Try and recognize if you see some pattern.

As for the kap tests, they are always deemed to be on the tougher(read unrealistically tough) side of the spectrum. In my personal experience, RC and CR in the verbal sections of the kap's are quite confusing and much tougher than those you can expect to encounter on the actual GMAT. Having said that, kap's are excellent stamina builders in that, the difficulty level of the questions warrant you 100% concentration 100% of the test time, thereby preparing you well for the test day pressure .