Why does the capacitance of ceramic capacitors change with test measurement voltage change?
That's the DC Bias Characteristic and is the reason for derating when a ceramic capacitor is used. That means that among ceramic capacitors, the capacitance, especially of capacitors classified as high dielectric constant (B/X5R, R/X7R characteristic), may differ from the nominal value when a DC voltage is applied. 



The effect is linked to the crystal structure of the capacitor's dielectricum.

For more details regarding this topic there's e.g. a nice article on the Murata website:
https://www.murata.com/support/faqs/products/capacitor/mlcc/char/0005