Question: Why do we see 802.11bg users in user session reports?
The basic explanation for the users you see being classified as 802.11bg on your AMP is that these are user sessions that for whatever reason AMP was unable to fetch a true connection mode type (802.11a/g/n) from the controller (could be caused by incomplete user association, very short user session (less than poll period), and etc.).
Normally AirWave is able to retrieve the connection type after one or two polling periods depending on what state the user is on the controller at the time of the initial poll where we discover the user. However, in the event that an actual connection mode is unavailable, AirWave will "fill in the blank" with the radio type of the AP that the user is connected to. For example, in some cases, Aruba AP65s are listed with 802.11bg radios and AirWave is using this to fill in the blank for some users.
One specific case that's known to cause this is when a user passes by an AP and her device (mobile phone likely) attempts to associate to the wireless network, but the user passes out of range before it can be completed. This is lot more likely to happen with 2.4 GHz users as the range for beacon detection is very long. But overall you should see a very small percentage of users classified as such, and ultimately these users should be properly classified (and they were when I checked) normally and will show up under the correct connection mode type counters in the reports.
This is intended system behavior and is unlikely to change unless we find that a large percentage of your users are being misclassified.