Low Pass filtering in Delta Sigma modulation
If you averaged 4 samples from a standard ADC connected to a source that has band-limited (but spectrally flat) white noise then you would increase the resolution by 1 bit. This is the standard approach.

But, you are talking about a sigma delta ADC and the returns are much better than one-bit per four samples because this type of ADC produces quantization noise that is much greater at higher frequencies thus, averaging is much more effective.


(source: eet.com)