ISO file creation -- procedure

Hi All,

Am creating ISO file for same directory two times and both has different md5sum, Why does it so ?!

Am pretty sure that, there is no change in that directory between the ISO creations, So what could be issue ? Or is there any misunderstanding in the process.

mkisofs -J -R -otest1.iso ./testdir
mkisofs -J -R -otest2.iso ./testdir

Expecting both the md5sum to be same !! but it is not ??

$ md5sum test?.iso
2a1001de29b75013d1bb5a8a787cf2ef  test1.iso
d01b9b1120ec6662d582dc3af6f94338  test2.iso

Side Note: I have related question posted already here, which dont had answer, so tried other ways and the above is one of the doubt in other way... edit volume id of an iso file ?