system is swapping (not actively), why?

good morning to all and happy new year!

i am facing the following situation; there is a server which is using 2-5GB
of swap very often (almost all the time) and i cannot figure out why. i can
provide any information you find useful and i will start what i think is
more relevant to my 'problem'.

-os -> CentOS release 5.6 (Final)
-the server is a dedicated db server with nothing else running there
-postgres version -> 9.0.5
-shared_buffers -> 15 GB

-vm.swapiness = 0

free -m
total used free shared buffers cached
Mem: 64445 61505 2940 0 159 59989
-/+ buffers/cache: 1356 63089
Swap: 32767 1868 30899

vmstat -n -S M 10 5
procs -----------memory---------- ---swap-- -----io---- --system--
-----cpu------
r b swpd free buff cache si so bi bo in cs us sy id
wa st
4 0 1867 1533 161 61345 0 0 682 164 0 0 9 1 86
4 0
1 0 1867 1286 161 61643 0 0 17932 3224 9776 9957 5 1 94
0 0
3 0 1867 1119 161 61802 0 0 12877 13080 3297 2580 5 1 95
0 0
2 0 1867 929 162 62006 0 0 12386 9406 5871 5749 5 1 94
0 0
3 0 1867 652 162 62190 0 0 13134 4912 6531 6157 8 1 91
0 0

as you can see from above the si/so are always 0 (i have run it for many
iterations and they are always 0) so the system is not actively swapping,
but still, why does it use the swap?
i have found online in some relevant threads/articles that there are some
operating systems that prefer to swap out the memory from the shared_buffers
that is not used. as i can see when inspecting the pg_buffercache, there are
3-5 GB with usagecount=0. could this be the case?

thx in advance for any advice

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