I have wondered why the RA/RT replenisher seems to work well without starter. Perhaps it has some restrainer in it already. I have looked at the starter label and it says it contains chloride (usually potassium chloride) and other ingredients such as carbonate (pH adjustment?). IIRC Kodak says to make a working solution out of the replenisher, you must add water and starter.
At the lab I work at, at one time we used Trebla replenisher to make working solution. If you didn't add the starter to the replenisher when making the working solution the control strip would give an out-of-control reading. RA-RT replenisher does not seem to have this problem. But just what is the case with the RT/LU used by the OP?