And radalt don't lie.
Hmm. If avionics were that infallible, avionics specialists would all be out of our jobs. Radio Altimeters are pretty good at working out how far is the huge mass of planet Earth below the aircraft. When responding to a target as small as another aeroplane there'd be considerable doubt.
What is interesting is that you got a -20 on TCAS and 1800 feet on the Rad Alt. The TCAS system gets its information from the two aircraft exchanging their barometric altitude data over the Mode 'S' link, so there is one discrepancy to keep avionics people interested, if not very busy. One system is definitely telling porkies...
Pressure altitude transducers aren't perfect but they're all calibrated to the same laws (These days: but it wasn't always so...) You don't give your actual altitude or aircraft type - which would give a clue as to the altimeter in use - but up around 30,000 feet, the certified accuracy of a typical altimeter transducer would be around +/- 300 feet. So, if a lower aircraft is 300 feet above assigned and a higher aircraft is 300 feet below you still have 400 feet seperation in an RVSM airway, or 1,400 feet in non-RVSM airspace.
On, lets say a B767, your 200 feet difference is no cause for concern. Its within the system's RVSM certified limits.
RVSM seperation of 1,000 feet only became practical since the eighties when altimeters became mose accurate. Even then, aircraft still have to be certified as RVSM capable. We can't have any ancient 707 freighters, equipped with pre-historic "Kollsman Integrated Flight Instrument Systems" (KIFIS - anyone remember that?) mixing it with modern aircraft types can we?