Hi Kreol,
You ask an important question, although the answer will vary somewhat from Test Taker to Test Taker, and each answer is a combination of a lot of detailed 'pieces.' Without going into every little detail, here are the major issues:
1) The realism with which each Test Taker works through his/her CATs. Test Day is a specific 'event', made up of specific details that you CAN define... so you SHOULD be able to prepare for them. Most Test Takers don't consider the details (much less properly train to deal with those details), thus they are not properly prepared for Test Day.
2) There are a number of lower-quality products on the market that a certain number of Test Takers use; that lack of quality/realism impacts practice scores and gives a false sense of readiness.
3) The Verbal section of the GMAT is just as predictable and pattern-based as the Quant section is, although any Test Takers find it easier to deal with Quant patterns than Verbal patterns. For example, the process that you go through to calculate 1+2 is the same general process that you go through to calculate 123+456 (it's just a little more work to do the second calculation and the digits are different). In that same way, the logic behind 'X causes Y' in a CR prompt is the same as the logic behind 'A causes B.' You just might have to do a little more work to sort through the details because the words are different.
This is all meant to say that you CAN train to score at a higher level in ALL areas of the GMAT, but a certain responsibility falls on the Test Taker to train in the proper way.
GMAT assassins aren't born, they're made,
Rich