OS X Finder sorting by date shows some files in incorrect order
Caching.

Not all Finder dialogs use the latest information available for a file to improve performance. Information such as that is loaded asynchronously, but this sometimes fails and doesn't show the correct information.

If a directory is taking too long to load, Finder may sometimes prefer to load partial information rather than not listing the directory contents in a timely manner. If this data is never loaded, it is never cached and is consistently listed incorrectly in certain circumstances.

This can usually be fixed by remounting the partition (if applicable) or restarting.