Why are external HDD enclosures advertised as 3.5"

[Kenneth Burgener]

I was looking for a simple external HDD enclosure and I couldn't
remember, off the top of my head, if my standard desktop hard drive was
the 2.5" or 3.5" size.  So I pulled out the ruler, and measured the
drive.  It is roughly 5.5" x 4" x 1".  It would seem to me that the 3.5"
size is what I am looking for, but the drive is not 3.5" wide.  Why?