Why does Outlook show a different datetime value for a file than Explorer does?

While inserting a file (ReportRuer.exx) in an Outlook email, I noticed that the datetime it displayed for a file I had just updated was outdated. In Explorer, it shows the correct datetime (Outlook dialog on top, Explorer on the bottom):

Why the difference? My theory is that the Outlook value is "Date", meaning when the file was first created, whereas Explorer shows the more meaningful "Date modified" - but why would Outlook do this? Isn't date modified a more useful date to show than creation date?

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