While it is true that the GMAT prep questions were once used on actual GMAT tests, the test does evolve/change slightly over time. For instance, many say the quant has become much more difficult vs the questions from GMAT prep (which by now are many years old). Specifically for your question, GMAC did release a press statement saying it's aim was not to test only esoteric idioms but rather to make sure the test taker can pick up on the meaning of the overall sentence. If that statement alone encompasses all possible tweaks GMAC has made to SC questions is hard to say. Nevertheless, I wouldn't worry too much - as long as you are know your basics and are "in the zone" you should do much better on SC this time around.
Good luck, and don't hesitate to ask any more questions
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