Advice and answers from the PPC Entourage Team

If you have a target with a high ACoS, and you've lowered your bid price to below what Genius Bid would recommend in order for you to meet your Target ACoS, Genius Bid may suggest an increase based on the bid price you'd need to set to hit your target.

In short, you may have cut your bid down too far, and Genius Bid has determined that you can slightly increase it and still see a lower ACoS.