Based on the results of your walk, I would say it's reports all of the instances on that particular controller...and I say this because only one instance is reported. I would think that if you have multiple WLANS on that WLC, you would get multiple instances reported back, so, (maybe, like you) I'm confused by the description in the MIB object where it states:
"No of Mobile Stations currently associated with the WLAN."
which to me looks like "the WLAN" is used in the singular.
At this point, I think the best thing to do would be to open a TAC case with all of this info, and we can get with our developers for confirmation.
Hope this has been somewhat helpful, and please rate these posts.