Question About WAL filename and its time stamp

Hi,

From the pg_xlog folder, I found some files with interesting time stamps: older file names with newer timestamps, can you please advise why?

Set 1: How come 0000000400000F490000008D is 10 minutes newer than 0000000400000F490000008E?
-rw------- 1 111 115 16777216 Sep  4 15:28 0000000400000F490000008C
-rw------- 1 111 115 16777216 Sep  4 15:27 0000000400000F490000008D <===
-rw------- 1 111 115 16777216 Sep  4 15:17 0000000400000F490000008E <====
-rw------- 1 111 115 16777216 Sep  4 15:26 0000000400000F490000008F
-rw------- 1 111 115 16777216 Sep  4 15:27 0000000400000F4900000090

Set 2: why files,  0000000400000F48000000FD,  0000000400000F48000000FE and 0000000400000F4900000000, are not reused?
1) -rw------- 1 postgres postgres 16777216 Sep  4 23:07 0000000400000F48000000FA
2) -rw------- 1 postgres postgres 16777216 Sep  4 23:08 0000000400000F48000000FB
3) -rw------- 1 postgres postgres 16777216 Sep  4 23:09 0000000400000F48000000FC  <===
4) -rw------- 1 postgres postgres 16777216 Sep  4 14:47 0000000400000F48000000FD  <====
5) -rw------- 1 postgres postgres 16777216 Sep  4 14:46 0000000400000F48000000FE
6) -rw------- 1 postgres postgres 16777216 Sep  4 14:46 0000000400000F4900000000

regards