Base of CAS is 2500ft, ILS traffic is at 3000ft, you're trundling along at 2200ft underneath CAS but your transponder overreads by 200ft. Suddenly you hit turbulence and climb 200ft. Your transponder is now showing 2600ft (within tolerance, perfectly legal). ILS traffic now gets a TCAS RA. That's the kicker.
TCAS will sometimes - not often, but occasionally - set off RAs even on traffic transponding at 2400ft outside CAS, even when both appear straight and level.
Pilots and airlines do not like TCAS RAs one bit, especially in the approach phase.