Here is the actual birth record of Wilhelmus and it is a little difficult to read the actual age of Johanna the mother, but i beleive the way he writes his V's that it reads Vier en Dertig = 34 years old. So she was probably born in 1817. Johanna was not married, and the father is not listed.
I couldn't find a Johanna Maria born from 1815-1817, but there was one born in Erp, on 9 May 1814. So she would have been 37 not 34 when she had Wilhelmus. It is possible they had her age wrong as she wasn't there when they supplied the information. If thats the case, then as luck would have it Johanna Maria was herself also a product of an unwed mother, father not listed.
The Mother is listed as Maria Hendrik, the birthdate seems to be 9 May 1814 born at Erp, Noord Brabant. The record is in French and my French isn't so good perhaps someone else can translate.