OSM-Carto does some heuristics on size in that case, the bigger/surrounding simple polygon makes the pattern, the smaller/contained one makes the colour. If you want to map a swamp you just have to draw a lake in a forest. Not that this is friendly to other consumers, people use that to map “Hutweiden” (pastures below trees) eg.
I am but in the absence of other evidence I would tend to assume it’s the user…
That’s amazing!
I did have some complicated relations myself but not like that one!
Well, not quite all of them. Until someone fixes it, this one is a counterexample, and also shows that the Carto rendering doesn’t handle multipolygons as inners of other multipolygons. At least that means that I am in good company if my renderer cannot handle them either!
I fail to see what the error is supposed to be in this example. You’ve simply created two independent multipolygons however the inner MP is -not- a member of the outer one.
Widening our views a bit to the whole of England (I do not dare tossing the entire UK at my poor iMac from 2014!) we find this one, apparently created by someone trying to deflect attention by calling themselves SomeoneElse! 
(You can probably find one created by me, somewhere — I thought this was supposed to work, and wrote my own multipolygon importer to handle it. I only noticed what the wiki says as I was fixing an unrelated bug… And as we know, the persons who edit the wiki, the persons who map, and the persons who write the software sometimes seem to be disjoint sets, so I thought I’d better ask!)