Zend certified PHP/Magento developer

Why does Windows Explorer change the ‘Folder Modify Time’ when first accessing a folder after a robocopy?

I am attempting to use Robocopy to mirror a directory structure (/e) and keep created/modified/accessed times. This should be fairly simple with ‘/copy:DAT /dcopy:DAT‘ and appears to preserve the file and directory times as expected.

The problem is, the first time I open a folder in explorer after robocopy finishes, the ‘modified’ time changes to the the current time. Subsequent accesses do not show this behavior.

What could be causing this behavior?