In my opinion, the technical writer got it backwards and the proofreader didn't catch the error.
On every vehicle I have ever had, the SET button not only set the cruise control speed, but also when engaged reduced the set speed by one mph. Likewise, on every vehicle I have ever had, the RES (resume) button not only resumed the vehicle to the most recent set speed, but also when engaged increased the set speed by one mph. That includes the Beaver Marquis I currently drive.