This isn’t always true! Last week our party of 9 had ADRs at Sci-Fi for parties of 6 and 3, but we really wanted be seated as 5+4. I checked in in person to tell them, and the CM told me to use mobile checkin and reduce the 6 to 5 and increase the 3 to 4. I checked in the larger ADR first with no problem reducing to 5, but when I checked in the smaller group it would not let me increase to 4! I had to return to the podium when they called the first one, to make sure we were seated correctly.