carto - Choropleth Mapping Using Quantile Method in CartoDB

My understanding of choropleth maps that use the quantile method is that they will assign an equal number of items to each bucket (see http://www.ncgia.ucsb.edu/cctp/units/unit47/html/comp_class.html).

A recent map I made with CartoDB generated a map that did not do this. Instead, most items were assigned to one bucket.

Is CartoDB using a different type of quantile method than what I described?