In the picture there are two measurements. The one is of a single 10uF capacitor as shown in the picture. The other measurement is of the 3x 3.3uF capacitors in parallel as shown in the picture below it. These three capacitors in parallel have about the same nominal capacitance as the single 10uF one. In the plot on the right in the picture there are two traces, these are both impedance magnitude traces (there is no phase data.) The upper trace is of the 10uF capacitor and the bottom one is of the three in parallel.
It is clear that the impedance characteristics of the two are different, even though the size of the capacitance is about the same.
The equivalent cct of a non-ideal capacitor is a series RLC circuit.
The answer I think is correct:
I think the answer is because inductors in parallel plays a role in determining the actual impedance of the capacitor. When there are 3 capacitors in parallel, the series inductance also come up in parallel. When inductors are combined in parallel, the net inductance becomes very small. When the net inductance becomes small, the net impedance also is small. This is why the impedance of the capacitors in parallel is slightly less than the single equivalent capacitor.
Please let me know if I am correct and if there is any additional information I have left out (like the role of the resistance.)