Sounds like the Ubuntu install was started whilst Windows was loaded,
i.e. a WUBI install rather than a full install involving resizing the
Windows partition. You still end up with a dual boot but the Ubuntu
'partition' is actually just a large file within Windows. Looks just
the same as a full dual-boot but has a slight performance hit though
easier to remove ( i.e. you can just uninstall from Windows like any
other program ).

As for systems without OS's - my UK supplier has these all the time,
desktops and occasionally laptops, which are a little cheaper than
equivalent specification systems with Windows already loaded. Remember
an OEM will not pay the same as we would for their Windows licences
but would have to purchase them thousands at a time. Also depending on
prices it can be cheaper getting the parts individually and building a
desktop from scratch. When asked to build a system I'll sometimes go
for the individual components, sometimes a bundle deal on motherboard
/ CPU / RAM plus case etc, sometimes a ready built one without OS. All
depends on what's available at the time :-)