mem usage

Subject : Linux memory allocation issue
My centos 5.4 has 6G ram and shows 4.5G is FREE

-bash-3.1# free
           total       used       free     shared    buffers     cached
Mem:       6222804    1662404    4560400          0       5600      79560
-/+ buffers/cache:    1577244    4645560
Swap:      2031608         76    2031532

But meminfo shows that 5.8G is full!!!

-bash-3.1# cat /proc/meminfo  | grep -i Comm
CommitLimit:   7632128 kB
Committed_AS:  5865924 kB

And hence, radius server repors
radiusd: Cannot allocate memory
Why the output of meminfo is differ from free? What should I do now?