Why are download times not shown accurately?

I just downloaded the update titled "DirectX 9.0c End-User Runtime" from
Windows Update.

The page that lists the available updates said it would take 3 minutes.
However, once the update had "finished downloading" and "started installing",
it insisted on downloading over an hour's worth of additional material before
installation would complete. The displayed time ought to have included that
material.

In case it matters, I'm running Windows 98 SE on a Compaq 1400 laptop using
its built-in dialup modem. (But the Microsoft site is not keeping the
"incoming data" light always-on, from which I infer that the Microsoft site,
rather than my modem, is the slow step that determines the download speed
anyway.)