i have tested the same thing, and you are right!
But also when i copy the file to an ext3 (on a linux system) and modify the file, i see the same thing happening.
As a last test i copied this file to mu C:\TEMP\ folder, and edit it......
Same result again! (The dates after 'Created:', 'Modified:' and 'Accessed:' are the same (and equal)
This might also be caused by the application i used to test this (LibreOffice Writer)