I understood after I watched the brilliant Weber Auto video on it, published 5 years ago[0]. I liked it so much I "gently" encouraged my partner to get a Yaris hybrid, which gets about 69mpg average. It's not unusual to see 80+ on individual trips. She was a little sceptical about having an auto at first, because us Europeans still think it's 1980 and that manuals are superior, but she came around after a few minutes…
I was curious too, I have a manual, petrol engine, Citroën C4 Picasso (called a minivan in English I think? but then I didn't think a Golf GTI was called a minivan so I'm not sure) from 2011 that's not hybrid at all and that's about what I get (7 L/100 km). It's comfortable to be in but not interesting to drive at all though.
I get a very consistent 4,3 L/100 km (54 mpg) on my diesel C4 (basically the same size as a Golf GTI) from 2013 and I like driving this one.
edit now that I've read the sibling comment, Google uses US gallons for its conversions and that's what I've used here.