The whole 128-bit value is encoded in big endian. But the fields decompose into something that's not. If you search the page for "UUID record layout" you'll find the timestamp as I've described it.
Last week I replaced an old broken window. I took a fragment of glass to the local glass company, and asked whether they cut custom glass. The receptionist said, "No, we do not." After some back-and-forth, I realized she thought I wanted to provide my own glass. I clarified, "I am just trying to replace a broken window." She handed me a form, and the next day I had a correctly shaped glass pane.
If UUIDs contain time information, then they can be sorted by time. The details of the encoding, while important for actually implementing the sorting algorithm correctly, don't really seem relevant when reasoning at a high level?
