The Royal House Act 2002 was perfectly gender neutral:
The monarch =
De Koning / De Koningin
The consort =
prins (prinses) der Nederlanden
The Heir =
De Prins (Prinses) van Oranje
The consort =
prins (prinses) der Nederlanden
The children of the monarch or the Heir =
prins (prinses) der Nederlanden
Unlike all consorts to a Prince of Orange before her, Máxima never became
Prinses van Oranje because that title was made gender neutral and attached to the person of the Heir exclusively.
The same happened to the title of the consort to the monarch. That became gender neutral too: no matter a male or a female, he/she will be
prins (prinses) der Nederlanden.
What happened in 2013? The Government and the Parliament, faced with their very own Act, started to walk on two wobbly ropes. At the one side they wanted to respect the law which says that Máxima is
prinses der Nederlanden, like Claus, like Bernhard, like Hendrik. At the other side they found it "undesirable" that Máxima was treated differently from other female spouses to Kings. So a typical -ugly- Dutch solution was found, a mix of the formal title (prinses der Nederlanden) with the informal form of address for a female spouse of a King (koningin Máxima):
It is not:
H.M. koningin Máxima der Nederlanden
But it is:
H.M. koningin Máxima, prinses der Nederlanden, prinses van Oranje-Nassau, mevrouw Van Amsberg