My grandmother’s official name was Catharina Flooren, but she was known as “Toos.” That’s a common derivative of Catharina, which came about via Catharina > Cato > Toos. Other examples of nicknames that might seem surprising are Mees for Bartholomeus, Elen for Aleida, and Nel for Petronella or Cornelia.
Especially before the introduction of the civil registration, you can find people in records under different variations of their names.
To find out other forms of a name, you can check the Corpus of First Names in the Netherlands. The “Verklaring” tab will often provide a link to the root form of a name.