I honestly do not know — I indulged in some speculation at the end of the article but am not qualified to say with any certainty. Again, I would be willing to wager that if the visualizations were redrawn taking into account homonyms, we would not see such disparity.
The question of what languages are best compared with English is difficult in its own right and the subject of some debate — a friend who works at duolingo pointed me here:
Duolingo is the world's most popular way to learn a language. It's 100% free, fun and science-based. Practice online on…www.duolingo.com
I believe French and German are relevant comparisons (Japanese I had done anyway and thought would be fun to include), though I admit visualizing other languages, particularly Danish, Swedish and Romanian, as Sam Cel Roman suggested, would be telling and help answer more of these questions.
Here is a quote from an article by Barbara Ann Kipfer:
The richness of the English vocabulary and the wealth of available synonyms means that English speakers can often draw shades of distinction unavailable to non-English speakers. Modern English has an unusually large number of synonyms or near-synonyms, mainly because of the influence of very different language groups: Germanic (Anglo-Saxon and Old Norse, the main basis of English), Romance languages (Latin, French), and Greek. There are many sets of triplet synonyms from Anglo-Saxon/Latin/Greek and also Anglo-Saxon/Norman French/Latin-Greek like cool-calm-collected and foretell-predict-prophesy. Peter Mark Roget’s intent was to catalog words by meanings, but he ended up inventing a new form of reference book.
I think it’s reasonable to think this explains in part why english is more centrally clustered, but again, I am not an expert in this field. One of my goals in writing this was to inspire people reading to shed light on these results and offer explanations.