B737 CREW CLBS TO FL270 FOR SMOOTHER HOLDING; BUT ARE INSTRUCTED TO DSND TO FL260; BECAUSE ANOTHER ACFT IS ALREADY IN THE HOLDING AT FL270.

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ENRTE FROM TPA TO HOU WE; AND EVERYBODY ELSE; WERE SEARCHING FOR SMOOTH CRUISING ALTS. AS WE CONTINUED PAST MSY; WE WERE CLRED TO FL250 (OPPOSITE DIRECTION) TRYING TO STAY IN SMOOTH AIR. HOU WAS FOGGED IN; AND WE WERE TOLD TO HOLD AT THE KUGLE INTXN. SINCE FL250 HAD NOW BECOME CONTINUOUS CHOP; WE DISCUSSED A DIFFERENT ALT WITH THE CTLR. SHE SAID HIGHER WAS BETTER IN THIS AREA. FO TOLD THE PAX AND FLT ATTENDANTS ABOUT THE FOG AND HOLDING. ENTERING HOLDING AT THE KUGLE INTXN ON THE TRINITY 1 ARR AT FL250; I (PNF) SWITCHED TO #2 COM TO TALK TO DISPATCH. I NOTICED THE FO TALKING ON THE RADIO. HE DIALED IN FL270 IN THE ALT WINDOW; AND I COULD HEAR HIM SAY 'CLB TO FL270.' WHEN I RETURNED TO #1 COM; FO STATED WE'VE BEEN CLRED TO FL270. AFTER MORE THAN 1 COMPLETE TURN IN HOLDING; CTR ASKED WHAT OUR ALT WAS. I SAID FL270. HE SAID 'YOU ARE SUPPOSED TO BE AT FL260?' FO REPLIED THAT WE HAD BEEN CLRED TO AND READ BACK FL270. CTR SAID 'DSND TO FL260;' AND WE DID. REST OF FLT WAS UNEVENTFUL. MULTIPLE ALT CHANGES HAD ALREADY FLOWN OPPOSITE DIRECTION ALT FOR QUITE SOME TIME MAY HAVE CONTRIBUTED TO CONFUSION AND THE LACK OF FL270 SEEMING LIKE AN UNUSUAL ALT. (RED FLAG) 1 PLT OFF OF #1 COM ALSO LOSES THE BACKUP FEATURE OF 2 PEOPLE HEARING CLRNCS. ALSO; ANOTHER FLT ON SAME RTE; POSSIBLY CONTRIBUTING TO CONFUSION WITH SIMILAR FLT NUMBER. SUPPLEMENTAL INFO FROM ACN 569316: ENRTE FROM TPA TO HOU; AS WE APCHED KUGLE INTXN (APPROX 50 MI OUT) WE WERE GIVEN HOLDING INSTRUCTIONS DUE TO HOBBY VISIBILITY BEING LESS THAN 1/4 MI. CLRNC WAS COPIED AND ACKNOWLEDGED. WE WERE AT FL250 AND RECEIVING A BUMPY RIDE. THE CAPT ASKED IF A LOWER ALT WAS AVAILABLE FOR HOLDING DUE TO THE ADVERSE RIDE. THE CTLR STATED DUE TO ARRIVING TFC FOR IAH; WE COULDN'T GO LOWER; BUT HE WOULD CLB US TO POSSIBLY IMPROVE THE RIDE. THE CAPT WAS OFF TALKING TO DISPATCH (MY LEG); WHEN THE CTLR CLRED US TO CLB TO FL270. I READ BACK 'CLB TO FL270' AND COMMENCED THE CLB. WITHIN A MIN; I CALLED 'ENTERING HOLDING;' AND HE ACKNOWLEDGED. THERE WERE OTHER ACFT HOLDING; INCLUDING A JET FLT NUMBER CLOSE TO OURS. AFTER 1 COMPLETE TURN IN HOLDING AND STARTING THE NEXT; THE CTLR ASKED US WHAT ALT WE WERE AT. I TOLD HIM FL270; AND HE SAID WE WERE SUPPOSED TO BE AT FL260 AND SHOULD DSND NOW. I BEGAN THE DSCNT AND ASKED HIM IF HE HAD US CONFUSED WITH THE OTHER ACFT. HE SAID 'NO;' AND THAT HE HAD GIVEN US FL260. I WASN'T GOING TO ARGUE AND SAID 'OKAY!' ALTHOUGH THE CAPT DID NOT HEAR THE CLRNC CLRLY; HE STATED HE WAS LISTENING TO VHF #1 ON LOW VOLUME AND HEARD ME REPLY 'CLBING TO FL270.' I FEEL CONFIDENT WHEN THE TAPE IS REVIEWED; IT WILL REVEAL WE WERE GIVEN FL270 AND THIS WILL MATCH MY READBACK. WE DID DISCOVER LATER THAT THE OTHER ACFT WAS AT FL270; BUT WE NOT SEEING HIM ON TCASII UNTIL AFTER WE TURNED OUTBOUND IN HOLDING AND SAW HIS TCASII TRACK TURNING INBOUND. ALTHOUGH I FEEL THIS WAS A POTENTIALLY DANGEROUS SIT; IT DID NOT DEVELOP TO SUCH. I BELIEVE THE CTLR CONFUSED HIS ACFT.