First off, please remember two very, very basic things.
DC will see every ounce of sales data. From online sales, overseas sales, trades, ect. We don't. They also know to the penny how much each title cost to produce, and they aren't the same. Various artists and writers earn in some cases very significant differences in page rates, to some that make minimum rates. That can have a significant impact as well on the profitability of a series.
One can't take, for example, the sales data from Diamond and use that as a 100% gauge of how a book is doing. There could be other factors that make enough difference in overall revenue to costs, that wouldn't make since just using Diamond (or any one) bit of sales data, and again you must have specific cost of each book as well.