I guess the difference is that those CTR stubs extend to the same altitude as the main CTR, and are controlled by the same ATC unit (Schiphol Tower), while a CTA would overlay the CTR to a different altitude.
Just guessing here, but by introducing a CTA between the CTR and the TMA they may have had to add an additional layer of ATC as well.
But as I said earlier, for us uncontrolled VFR flyers the difference between a CTR, CTA and TMA is really moot. Just look at the type of airspace involved, and look up the controlling authority if you want to go through. And if you can't go through, go around.