Significant swap-in and swap-out activity in spite of a lot of free memory

Issue

Significant swap-in and swap-out activity while there is several gigabytes of free memory available.
Notice the following excerpt vmstat:

2014-01-20 04:02:13 procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------
2014-01-20 04:02:13  r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st


2014-01-20 21:05:37  1  0 476328  54108   2448 8756568    0    0 21731   136 2127 6541  1  0 96  3  0
2014-01-20 21:05:57  0  1 476328  53688   2292 8752844    0    0 22492   592 2201 6623  1  0 96  3  0
2014-01-20 21:06:17  0  1 476328  48712   2420 8759916    0    0  9684   820 1957 6485  0  0 96  4  0
2014-01-20 21:06:37  0  1 476328  48740   2404 8760228    0    0  9054   137 1961 6470  0  0 96  4  0
2014-01-20 21:06:57  0  1 476328  50352   2560 8760524    1    0  8503   496 1889 6316  0  0 96  4  0
2014-01-20 21:07:17  1  0 476328  50844   1772 8755260    0    1 91440   915 3051 6803  1  1 96  3  0
2014-01-20 21:07:37  2  0 482108 3628104   1360 5192632    7   20 46243   703 2487 6990  1  5 90  4  0
2014-01-20 21:07:57  2  5 1499572 3951156   1028 4461972   31 15985  1089 17192 2033 6727  1  7 90  2  0
2014-01-20 21:08:17  0 17 1614756 4090840   1252 4383308  264 7302   423  8467 1373 5568  0  1 76 23  0
2014-01-20 21:08:55  0 27 1631652 4178880   1332 4367380   11 3910    32  3930 1134 4885  0  0 65 35  0
2014-01-20 21:08:57  0 29 1634348 4283284   1364 4365220   14 4997    51  5016 1140 4870  0  0 63 37  0
2014-01-20 21:09:17  0 36 1634312 4308844   1396 4365392   21 1591    34  1612 1132 4969  0  0 63 37  0
2014-01-20 21:09:38  0 36 1634304 4329148   1436 4365380   13  863    37   872 1132 4959  0  0 48 52  0
2014-01-20 21:09:58  0 32 1634300 4353208   1456 4365548   13 1147    23  1155 1133 5063  0  0 47 53  0
2014-01-20 21:10:18  0 37 1634296 4371076   1496 4362128   10  845    17   857 1135 5047  0  0 50 50  0