Thanks for pointing this out. I had actually been thinking that
we were talking about a timestamp in the metadata, but I see
now that it was the system file modification date.
The filesystem information is handled differently from information
actually stored in the image:
By default exiftool does nothing special to try to preserve the
filesystem attributes when writing, so the System:FileModifyDate
will get set to the current time when a file is edited. However, this
time may be preserved by using the -P option. Other
filesystem attributes (such as icon position, etc) may also
change, but if you really need to preserve things these types
of things, the -overwrite_original_in_place option may
be used for this.