ARM chooses the channel based on the interference index, not how many APs are on the same channel. There is other interference on the 2.4ghz band besides access points.
Use the command:
show ap arm rf-summary ap-name <name of ap>
to see what the interference index (intf_indx Total) is. Again, there is little room on the 2.4ghz band, so I would consider using 5ghz if possible.