I am not sure if I should put this question in the British titles or non-British titles section as it is a comparison.
Mbruno wrote "The blame lies solely on the sloppy drafting of the Royal Decree's text by whoever was responsible for it in the government."
Exactly. And Delphine's detractors would not be gnashing their teeth over her royal windfall today had more careful legal experts been used in the 1990's. But they were not, so there exists -no legal obstacle- to Delphine and her kids being granted equity with Astrid, Laurent and their kids.
First, the court decision conferring royal titles on Delphine and her children
relied exclusively on the 2015 decree about the title Prince/ss of Belgium.
Nothing in the succession law from the 1990s has anything to say about titles.
Second, what is the reason that royal watchers on the whole treat the Belgian royal decrees very differently than equivalent royal decrees and decisions in other European monarchies, particularly the UK and Spain?
Following the court decision, the internet was full of comments declaring that the 2015 Belgian royal decree was sloppily drafted as it failed to explicitly mention words such as "legitimacy" or "marriage" and saying that for this reason, it should not be construed as excluding extramarital children.
That is debatably true (although it is not that simple, as there are other ways of construing the text of the decree, as was discussed in the Belgian threads). But the British letters patent of 1917 never mention "legitimate" or "marital", either: only "children" and "son".
Now Know Ye that We of our especial grace certain knowledge and mere motion do hereby declare our Royal Will and Pleasure that the children of any Sovereign of these Realms and the children of the sons of any such Sovereign and the eldest living son of the eldest son of the Prince of Wales shall have and at all times hold and enjoy the style title or attribute of Royal Highness with their titular dignity of Prince or Princess prefixed to their respective Christian names or with their other titles of honour
Yet the phrasing of the British 1917 letters patent is never assumed to be sloppy, and commenters hardly ever claim that there "exists -no legal obstacle-" to a child born out of wedlock to a British royal becoming a Prince/ss (in fact, I have seen commenters claim that it is impossible). Why the difference in interpretation?
(For the avoidance of confusion, my personal opinion is that it is simpler to interpret the text of the British letters patent as granting titles to illegitimate children than to do the same with the Belgian royal decree. But it seems obvious that most commenters feel that it is the the other way around.)