The main issue are always bonuses. I'm pretty sure that there aren't any players in Europe, bar Messi, perhaps, with the base salary of 300k. But most of the bonuses are quite achievable — appearances, goals, trophies... Italy also underwent a tax reform and is now a tax heaven for footballers (hence the recent wave of high-profile signings). De Jong is reported on 10 + 6, De Ligt is on 12, but it's probably quite a bit less if he doesn't play, Eriksen's signing bonus is probably spread out through out his contract etc.
Edit: if anyone is interested — Italy doesn't have a significantly lesser % of tax from a salary, but footballers can pay a fix sum of 100k for all of their income that came from out of the country. So, the image rights etc., which are usually a part of their agreement with the club in one way or another, are basically tax-free (well, for those footballers, who earn millions or tens of millions on them).