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The reason for putting a small capacitor in parallel with the bulk is that large electrolytic capacitors have large ESR and low resonant frequency so you put a small capacitor in parallel for high frequency decoupling.
As ptmsl said, large electrolytic (polarized) capacitors are only capacitive at low frequencies. As frequencies get higher and higher they actually start behaving more like inductors! So if to have capacitance at higher frequencies you use smaller capacitors in parallel (to add to the capacitance). By the way, these smaller capacitors are usually of different type (for example ceramic) so they have much lower equivalent series resistance and higher resonant frequency (they are behave as capacitors up to higher frequencies).