In order to keep up the simplicity and ease of use of Instant On, we have opted not to allow for the manual channel setting. But rest assured, the solution is such that there is constant scanning and optimization being done to select the best channels
When there are multiple other 2.4GHz networks in the area, why would ARM choose to use a 40MHz wide channel in 2.4GHz that interfers with the neighboring networks and causes the neighboring networks to interfer with the AIO AP? That doesn't make sense.
If there is interference detected, ARM will adjust accordingly. As each environment is different, it is hard to point out the exact reason this might be happening in your case. If this problem persists, please contact support and give them details of your network and environment.