Cisco Wireless :: 5508 - Not All 5 GHz Channels Being Used

I've got a deployment of 1142N APs using a pair of 5508 controllers running 7.3.101.0.  We're in the United States and the country is set to "US".  I've added the UNII-2 channels to Auto-RF on the controllers via "config advanced 802.11a channel add 100" and so forth.  According to the output of "
show advanced 802.11a channel", I've enabled every 5 GHz channel that's legal for use in the US:
 
802.11a 5 GHz Auto-RF Channel List
Allowed Channel List......................... 36,40,44,48,52,56,60,64,100, 104,108,112,116,132,136,140, 149,153,157,161,165
 
My confusion comes because the automatic assignment of 5 GHz channels is heavily biased toward just a few of them, with most of the available space being ignored.  Here are the 5 GHz channel assignments from a large, single-story building with 31 APs:
 
(qty 2) 36
(qty 6) 40
(qty 5) 44
[ code].....
 
A total of nine channels are used (two of them only once) while twelve other channels are ignored completely.  Many of these APs are close enough to hear one another, so assigning seven of them to ch. 48 makes no sense.
 
Looking at my campus as a whole, I've got numerous APs using channels 36, 40, 44, 48, 149, 153, 157, 161, and 165.  I've got at least one each using channels 52, 56, 60, 64, 112, 116, and 132.  Nothing at all is on 100, 104, 108, 136, and 140.  I am using 20 MHz wide channels, between, so all are independent.
 
Can anyone shed light on A) why my distribution is so heavily skewed toward the first nine channels, and B) how I can learn whether certain channels are being avoided due to the presence of radar, etc, on the frequency?  I'd like to take full advantage of 5 GHz by having as little channel re-use as possible.