I created these two zip files about 24 hours apart.
$ ls -l /tmp/*.zip -rw-rw-r-- 1 scott scott 1401289 Mar 21 20:11 /tmp/inc1.zip -rw-rw-r-- 1 scott scott 1401289 Mar 22 19:29 /tmp/inc2.zip
You can see they're the same size, because the directory being zipped hasn't changed.
However, when I run md5sum on them, I get different results.
$ md5sum /tmp/inc* 95b1c0b32cc24a5554769797e032ec6e /tmp/inc1.zip 80765ac3fc7f488243bdc1c06902e33c /tmp/inc2.zip
Why is this?