Wrong. They use different kernel versions, with different kernel patches. And most importantly, the userland apps certainly differ here and there. The most important example is the Mandriva Control Center. It's task-oriented, making it far more friendly than searching for configuration tools by name - in particular, if you have a localized system, where translations are often arbitrary and non-intuitive.
For specific examples, check out Mandriva's wizards for video cards, disk partitioning, network setup, network sharing. Now try to setup those things under Ubuntu without hitting the Ubuntu forums first.
That said, network card compatibility is pretty much hit or miss, as they often depend on binary blobs (either proprietary or windows drivers) that break in different ways with different kernel versions. My dad's current laptop's wifi only works reliably with WEP, not WPA, while mine kernel-panics with WEP. I bet bugs would manifest themselves differently on Ubuntu.