Amps is a measure of current flow or volume. Voltage is analogous to pressure. Your 20A circuits are capable of supplying more current than your 15A components will draw. Think of this as excess supply. This is a good thing.
If you had 240 volts, with 120v components, that would be another story because voltage is pressure, then you would have a problem. As it is you have no problem whatsoever.
If you had 240 volts, with 120v components, that would be another story because voltage is pressure, then you would have a problem. As it is you have no problem whatsoever.