Not used to my newfangled Honda, why does it do this?

I've always owned manual transmission cars and just recently due to age and some other factors traded my last one in for a Honda. It's automatic obviously. My eyes still automatically look at the tach all the time though and I've noticed that when I go to pass someone on the freeway, it will rev up to about 2500rpms and then go back down. As if it is downshifting. Why does it do this?

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