This is a very interesting question.
There is an article called "Flemish given names" in Wiktionary. (http://en.wiktionary.org/wiki/Appendix:Flemish_given_names
) It says:
"The basic Flemish name stock is Germanic, sometimes in its early forms. Since Roman
Catholicism is the most widespread religion in Belgium this is reflected in the great number of 'Catholic' names of saints. Nowadays there are stark influences from the Netherlands, Germany (the Belgian royal dynasty is from Germany) and the French speaking Wallony. There is a huge overlap with the Dutch given names, although there are typical Flemish names that can't be found in the Netherlands. Some examples are the female name Goedele and the male name Lieven
The source of this preliminary list of Flemish given names is a booklet from 1902 added with modern names taken from books, newspapers and seen on television and Internet."
The main author of this article is: http://en.wiktionary.org/wiki/User:Alasdair
, so maybe you could ask him more about Flemish given names.
Given that, as far as I know, there aren't true orthografic difference between Dutch and Flemish, a Dutch "official" form should be the same of the Flemish one (thus not like English-William, Dutch-Willem, German-Wilhelm, Danish-Vilhelm, Icelandic-Vilhjálmur).
It should be something like a Catalan-Valencian difference.
Divergence could be possibly explained by different traditions or foreign influences between Netherlans and Flanders.