They're trying to bring the asking prices of the cars more in line with their actual selling price.

People who go shopping for cars online tend to limit their search to a particular price range. But yer typical GM model that ends up selling in one price bracket has a sticker price that's a good several thousand dollars higher. So people might not even put that particular car on their list just because the price after incentives generally isn't shown online until you do a little looking, and if you limit your search based on sticker price then the "appropriate" GM model wouldn't show up.

Instead, now the asking price is lower, so the GM vehicles slot into their theoretical spots based on a number that is closer to the actual selling price than some theoretical selling price that nobody ever pays because of one different incentive program from GM after another.