We've seen this in our high density deployments (250+ AP with 3000+ clients).
Some things to look at:
What power level are the APs at currently? Do they all settle to the minimum power level allowed? You might want to reduce the max and min levels. Don't just reduce the minimum because it can cause a 'super' AP where clients see that signal strength and attach to it. It is entirely possible the intf_index calculated by ARM is very high and the same channel may be the best channel based on the ARM algorithms. We had this issue until we lowered the max/min power levels down to 12/9 or so.
I don't like to put APs in a line down the hallway (if that is what you have) or where they can all 'see' each other without walls in between. Realize that ARM is taking measurements and these APs see each other with strong signals. This can cause them to turn down their power level as well as increase interference. To add insult to injury, clients are inside the rooms on the other side of the wall(s) which means they are seeing a reduced power level. Lowering the power level on the APs in the hallways will help with ARM channels/power but clients will have reduced performance because they are getting less signal strength.
For every deployment possible, I put APs in rooms with the users and keep APs from 'seeing' each other directly. This can require a few more APs but the performance of the entire system is improved.
By the way, are these 2.4ghz or 5ghz channels? 2.4ghz is much more likely to see this issue than 5ghz (number of channels, etc.).