Discrepancy between shared memory from /proc/meminfo and ipcs

Question

Running free -hm outputs:

             total       used       free     shared    buffers     cached
Mem:           15G        15G       389M       321M       169M        11G
-/+ buffers/cache:       3.9G        11G
Swap:         7.9G        80K       7.9G

The 'shared' column is the same as /proc/meminfo Shmem (332072 kb), but when I try to track down the processes that are using the shared memory with ipcs -m, it only shows ~24MB usage:

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status      
0x00000000 851968     dangets    700        7978224    2          dest         
0x00000000 458753     dangets    700        7978224    2          dest         
0x00000000 589827     dangets    700        7978224    2          dest         
0x0052e2c1 327684     postgres   600        48         5                       

Here's where I get fuzzier, so please correct me if I'm wrong.

I also looked for any tmpfs mounts, and it is only the /run mount, which du -chs /run says that it only is 123MB.

Can someone please explain or point me to documentation on what actually contributes to the /proc/meminfo Shmem field? Thanks!