QGIS How are data points of the same value classified for Equal Count (Quantile)?

it looks like the behavior you saw should be reproduced every time. I believe the code you found (and linked) is general and used for every type of classification which QGIS makes available in the symbology tab. It is reliant on the calculateBreaks() method which is defined separately for each type of classification.

Here is the link to the equal count (quantile) implementation of calculateBreaks(): https://github.com/qgis/QGIS/blob/master/src/core/classification/qgsclassificationquantile.cpp

The way these breaks are generated uses a static formula in the code linked above. Then, the code you found is used to assign each data point into a category between those breaks. All ties will still be between one break and another and will be sorted into the same place.

I'm not totally sure what the static formula is doing line by line or I'd give a better explanation, but it seems to be essentially following the steps described on statistics how to here: https://www.statisticshowto.com/quantile-definition-find-easy-steps/

Here's some insight: https://issues.qgis.org/issues/21451

But in short, items with the same value need to be assigned the same rank, meaning sometimes you'll get differing numbers in each quantile. Think about it this way. If you have 2 first-place teams there won't be any second-place team.

For my own use, I created a processing algorithm for this that adds and populates a ranking field. I've been meaning to add this to a GitHub repo, and just have. You can find it in its ragged glory here:

https://github.com/davidlgalt/locitools/

Disclaimer: When it comes to PyQGIS & GitHub I am still on the uphill side of the learning curve.