Existing YAML tests of update method include a upsertedCount field, e.g. here. The new tests for arrayFilters do not, e.g. here. The difference between these tests complicates the required test runner logic, which already has to manufacture this field for comparison (the write commands don't actually return upsertedCount, but rather a list of upserted ids).

It's possible that it was done this way to preserve compatibility with 2.4. In that case, the tests should all be fixed to account for the fact that 2.6 is now our minimum supported server version.