@greg the effective capacitance of a class II MLCC is set by how many electric domains you have available to be polarised by an applied electric field. however, if you apply a DC voltage over the capacitor, the electric field polarises a bunch of the domains, and they stop being useful. this causes the capacitor to have less effective capacitance to deal with the AC portion of the wave.
the effect can be significant. a 6.3V rated capacitor might lose 70% of its capacitance at 5V DC bias.