AN A320 FLT CREW EXPERIENCED AN RA WHEN FLYING 1000 FT BELOW TFC BECAUSE OF MOUNTAIN WAVE AND THE CHARACTERISTIC OF THE AIRBUS TO FLY SOFT ALTS.

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ZAB NOTIFIED US OF TFC AT 12 O'CLOCK POS; 10 MI AT FL370; WE WERE LEVEL AT FL360. TFC SIGHTED BOTH VISUALLY AND ON TCAS. ACFT WAS EXPERIENCING VERY SLIGHT MOUNTAIN WAVE. AT APPROX 4 MI AND CLOSING WE RECEIVED A TCAS RA 'DSND; DSND' WITHOUT ANY PRIOR 'TFC; TFC.' I DISCONNECTED BOTH THE AUTOPLT AND AUTOTHROTTLES; NOSED DOWN WITHIN THE ALLOTTED 5 SECONDS TO 1200 FPM DOWN; TFC CROSSED OVERHEAD WHILE WE WERE AT FL358.5; COMMENCED A CLB BACK UP TO FL360. WE TOPPED AT FL361.3 DURING THE RETURN TO LEVEL FLT. FO ADVISED ATC THAT WE HAD COMPLIED WITH A TCAS RA. CTR QUERIED BOTH ACFT AS TO THEIR INDICATED ALTS. (THE OTHER ACFT HAD NO TCAS INDICATIONS AT ANY TIME DURING THIS PERIOD). WE SHOWED FL360 ON CAPT'S PFD; FL359.8 ON FO'S PFD; AND FL360 ON STANDBY ALTIMETER. WE WERE ASKED TO CALL ATC ON ARR AT PHX. I SPOKE WITH MR X OF ZAB AND DESCRIBED WHAT WE SAW; WHAT WE DID; AND THE ALTS INVOLVED. MR X WAS UNAWARE OF THE AIRBUS CHARACTERISTIC OF FLYING A 'SOFT' ALT (+/-50 FT ON THE SET ALT) AND I SUGGESTED THAT THE EVENT MIGHT HAVE BEEN CAUSED BY A COMBINATION OF MINOR MOUNTAIN WAVE; SOFT ALT FLOWN; AND A SENSITIVE TCAS SYS. HE INDICATED THAT 'HEADQUARTERS' MIGHT WANT HIM TO FILE AN 'INCIDENT' RPT; BUT DEFINITELY NOT A 'PLTDEV' RPT. PAX AND CREW WERE UNAWARE OF THE TCAS EVENT.