Lately I've been noticing that my monitoring script which reads from /proc/meminfo and /proc/<pid> has been showing odd readings, where several hundred MB seem to be missing. htop is showing the same discrepancy:

 CPU: 36.9% sys: 7.6% low: 0.0% Tasks: 123; 1 running
 Mem:1750M used:1568M buffers:20M cache:87M Load average: 0.90 1.33 1.33 
 Swp:1787M used:184M Uptime: 1 day, 00:28:22
 PID USER PRI NI VIRT RES SHR S CPU% MEM% TIME+ Command
 3455 rena 20 0 1221M 312M 20464 S 14.0 17.8 1h20:26 /usr/lib/firefox/firefox
 1428 root 20 0 327M 100M 6236 S 7.0 5.7 1h28:52 /usr/bin/X :0 -auth /var/run/lightdm/root/:0 -nol
 14012 rena 20 0 630M 26400 5264 S 0.0 1.5 0:14.20 /usr/bin/python /usr/bin/sonata --toggle
 2562 rena 20 0 914M 21100 10904 S 0.0 1.2 3:59.50 xfce4-terminal --role "Terminal" --hide-menubar -
 2492 rena 20 0 379M 20528 3312 S 0.0 1.1 0:03.53 pcmanfm --desktop --profile lubuntu

Memory used is 1568M, but the top 5 processes aren't using anywhere near that: 312+100+26+21+20+20(buffers)+87(cache) = 586M. So 982M are missing somewhere?

OS is Lubuntu 12.04 AMD64.

contents of /proc/meminfo: http://pastebin.com/wSQ5Wst9