Some of the tests for analytic use of the regr_*** functions are testing nesting the analytic inside another Window Function. These use RANK as the Window Function. For some tests, all the results of the nested regr_*** function are the same – perhaps all NULL, or all 0.0000 or something. In these cases, RANK can rank them in any order and thus is non-deterministic.
All our regression tests need to be deterministic as we decide pass or fail based on a simple compare to a reference.