What you're seeing something is not being done right (Human error).
A example:
I created a file with vi you will see that at = mt:
truss -v lstat -t lstat ls -l count_files.awk
lstat64("count_files.awk", 0xFFBFF5B8) = 0
d=0x04010003 i=58647 m=0100755 l=1 u=0 g=0 sz=674
at = Feb 20 14:05:56 CET 2013 [ 1361365556.951290423 ]
mt = Feb 20 14:05:58 CET 2013 [ 1361365558.532478282 ]
ct = Feb 20 14:05:58 CET 2013 [ 1361365558.532478282 ]
bsz=1024 blks=3 fs=zfs
-rwxr-xr-x 1 root root 674 Feb 20 14:05 count_files.awk
Now I will open the file again with vi but exit doing :q!
Notice that at is not equal to mt.
truss -v lstat -t lstat ls -l count_files.awk
lstat64("count_files.awk", 0xFFBFF648) = 0
d=0x04010003 i=58647 m=0100755 l=1 u=0 g=0 sz=674
at = Feb 20 14:07:58 CET 2013 [ 1361365678.550493967 ]
mt = Feb 20 14:05:58 CET 2013 [ 1361365558.532478282 ]
ct = Feb 20 14:05:58 CET 2013 [ 1361365558.532478282 ]
bsz=1024 blks=3 fs=zfs
-rwxr-xr-x 1 root root 674 Feb 20 14:05 count_files.awk
Now let's make still another test:
Open the file with vi and exit but instead of :q! lets do :wq! (although we did not change anything, vi will still save the contents to the file)
truss -v lstat -t lstat ls -l count_files.awk
lstat64("count_files.awk", 0xFFBFF5F8) = 0
d=0x04010003 i=58647 m=0100755 l=1 u=0 g=0 sz=674
at = Feb 20 14:09:26 CET 2013 [ 1361365766.879205630 ]
mt = Feb 20 14:09:28 CET 2013 [ 1361365768.147368630 ]
ct = Feb 20 14:09:28 CET 2013 [ 1361365768.147368630 ]
bsz=1024 blks=3 fs=zfs
-rwxr-xr-x 1 root root 674 Feb 20 14:09 count_files.awk
As far as I know, at time changes when read is called and modified time when a write is done (write call ).
cat file > /dev/null
. Does the modify time change then?