I’m a bit baffled by the data you point to, the intelligibility is lower than I’d expected. Particularly from my own experience with fellow Scandinavians.
However, you should note that these data refer to spoken language and probably relates to a large degree to pronunciation. The written languages are very similar, and as amanset points out, the differences are so minor that ingredients lists on food packagings often use one language with only a few words written with alternatives.
I quite sure that if one had taken the same survey for comprehension of written lanugage rather than spoken, the scores would have been much higher. Let’s look at the vocabulary example in the next paragraph in the article you’re linking to:
It sure doesn’t look like three mutually non non-intelligible languages. At least not when written
ETA: It’s also not uncommon for e.g. people from the large cities to claim they can’t understand dialects in their own language. E.g. quite a few people from Oslo have a problem understanding people from, say, Voss, Stockholmers have problems with skånsk, and Copenhageners have problems with sønderjysk. In that case, it is the pronounciation