“Why does part of the Windows 98 Setup program look older than the rest?”

Well, this is something I never knew. Over on the retrocomputing section of StackExchange, someone asked why the second phase of the Windows 98 installation looked decidedly different from the third phase, even though they’re both graphical phases (the first phase is textual). The answer turns out to be both surprising, and entirely predictable.

The first phase is a DOS program called DOSSETUP.BIN, which is the infamous blue part of the installation. The second part, however, is what we’re interested in here, and if the first phase is DOS, and the third phase is Windows 98 itself… What do you think the second phase is running? Yeah, exactly.

Basically, because it is running under Windows 3.1 at that point.

The second uses this minimal Windows 3.1 to run a Windows 3 program, W98SETUP.BIN (specified as the “shell” in SYSTEM.INI).

This starts by copying more files to support all the information-gathering during setup, and various other niceties including the 3D look shown in your screenshot (the contents of the PRECOPY CABs); it ends by copying most of Windows 98, setting the system up so that it will boot Windows 98 from the target drive, and rebooting.
↫ Stephen Kitt

So, in order to install Windows 98, you first run DOS, followed by Windows 3.1, ending in Windows 98. I have no idea why this is so funny to me, especially since it fits entirely within expectations of how Microsoft does things.