Parallel Capacitor Conundrum
Capacitors eventually stop behaving like capacitors at high frequencies and exhibit resistive and inductive effects. The 100pF capacitor filters high frequencies that the 100nF capacitor could not.

This is shown clearly on the diagram below which shows the impedance (Red) of a 100nF capacitor and a 100pF capacitor (Light Grey top curve) versus Frequency. See how the 100nF capacitor starts to look inductive from about 10MHz whereas the 100pF is still capacitive out to 1GHz.



Link to the Kemet Spice Tool used for the capacitor analysis.