It's way more inland than the others on the list though. And there's some foothills in between there and the sea, Apennines, isn't there? So that would explain the difference. I'd expect there to be data for that one as it's not all about big cities.
According to Wikipedia, Luxembourg has 1725 hours of sunshine per year. Not much lower than the nearby capitals that made the list, but, as sillie points out, the inland location behind the Ardennes probably makes the difference!
1803 hours now according to Wikipedia, up from 1725 when the quiz was made, probably due to more recent data (1991-2020). It may be worth updating the quiz soon for the new reference periods, but Luxembourg isn't getting any closer to inclusion.
I tried Riga, Tallinn, Stockholm, Helsinki and Oslo and started wondering if there's a bug with the quiz or if I have problems with my laptop / internet connection since none of these worked. I was expecting that Scandinavia will have much fewer sunshine hours than, let's say, Paris. I still got them all in the end, but it was more challenging than I was expecting.
The polar nights are balanced out by the midnight sun. I tried them too, after I used the gulf-stream capitals up, but nope, the 16+ hour days in summer seem to balance the 6 hour winter days quite well....
Also, the number of hours of sunshine doesn't tell you how hot it gets. I'm pretty sure that one hour of sunshine doesn't feel the same in Dublin in January as it does in Nicosia, or other Southern European capitals, in July.
Fun Fact: Pittsburgh (which happens to be the closest major city to where I live) is widely considered the least sunny city in America. According to Wikipedia, it still gets 2,021 hours of sunlight--way more than even the "sunniest" city on this list.
Got Vilnius literally as it changed to 0 seconds. I'd already done Tallinn, Helsinki, Moscow, Minsk etc and they were wrong so I had just skipped over it.
This looked incorrect as more northenr cities are excluded. Here is the only Wiki page I could find: https://en.wikipedia.org/wiki/List_of_cities_in_Europe_by_sunshine_duration
This list does agree with the quiz though, doesn't it? Most northern capitals don't make the cut, for reasons already discussed in the comments. I generally got the data from the individual wiki pages for national capitals, which mostly give the same numbers as the linked page.
Very surprised that Paris was there yet no Scandinavian Countries. Vilnius only one that I missed.
Try this site
(If you are a plant, i'd be very surprised)