8. A CMB photon emitted by material within overdense regions will lose energy as it climbs out of its local potential well (via gravitational redshift). See Bothun, section 3.1.

Note that the treatment here is somewhat oversimplified: the scale corresponding to the fluctuations detected by COBE is very large (~1000 Mpc in comoving units) - much larger than any collapsed structures today. On smaller scales there are also effects other than gravitational redshifting which come into play - notably Doppler shifts due to peculiar motions, and temperature fluctuations arising from adiabatic compression of gas in overdense regions.