Thanks for doing the experiment.
When the UK (and PC!) is on summer time,
(auto detect) still gets UTC(0)/GMT, and not BST, so it's 1 hour out.
Could it be getting your PC's local UTC(+/-) time through the browser, even through the VPN's browser hiding I wonder.
Minor update:
I've just changed my PCs hour, it seems when the Look and Layout page is first opened it somehow captures your local UTC(+/-) time, and then clicking
(auto detect) just fills in what was captured.
So maybe it's only your own browser doing it all locally, which would mean it's not all that clever after all !
I'll have to think about it a bit more....