Confusion with ADC conversion in MCP3424
If the conversion is 18-bit, there will be 262144 levels.

However, it seems to me that on the ADC Pi board, one of the differential inputs of each MCP3424 converter channel is grounded ; so this actually halve the range, and the first bit of a word read from the ADC will always be zero. That's why the ADC Pi datasheet reports half the resolutions compared to those published on the MCP3424 datasheet (17-bit, 15-bit, 13-bit, 11-bit).

There is a relationship between data rate and bit rate because the converter has a delta-sigma architecture. There is an internal conversion/integration clocked at a fixed 983040 Hz, and this conversion "budget" can be used at your whim to achieve different data rate / bit-rate ratios with:

$$s_r = \frac{983040}{2^{b_r}}$$

Intuitively (and this is a gross mis-simplification), reducing the sample rate gives the ADC more time to extract the finer details of each sample.