The build events track the files that were changed that triggered the build, along with a load of other stats such as free memory, whether docker was running, etc.

I took a selection of builds that were triggered by the same code module (one that frequently changes to provide enough data) and compared models on just that, finding the same results.

This feels as close as you could get for an apples-to-apples comparison, so I'm quite confident these figures are (within statistical bounds of the dataset) correct!