A CGA LR35 CLBED ABOVE ASSIGNED ALT AFTER TKOF. THE RPTR CLAIMS THEY READ BACK THE ALT THEY THOUGHT THAT THEY WERE CLRED TO AND THE CTLR DID NOT CORRECT THE READBACK.

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STATEMENT REGARDING ALTDEV BY A LEARJET IN NOV 1996; AT APPROX XX25 LST; SW OF ZDC. DURING CLBOUT THE COPLT WAS FLYING AND MONITORING THE ATC RADIO WHILE THE CAPT WAS ENGAGED IN OTHER COCKPIT DUTIES. EITHER DURING CLB; OR SHORTLY AFTER REACHING AN ASSIGNED ALT OF 10000 FT; THE COPLT UNDERSTOOD THAT THE CTLR CLRED US TO CLB TO AND MAINTAIN 11000 FT. WE AGREED WITH EACH OTHER THAT 11000 FT WAS OUR NEWLY ASSIGNED ALT. THE COPLT READ BACK THE CLB CLRNC TO ATC; INDICATING OUR UNDERSTANDING THAT WE WERE CLRED TO CLB TO 11000 FT. WE SET 11000 FT IN THE ALT SELECTOR AND PROCEEDED TO CLB TO 11000 FT. UPON ARR AT THAT ALT; THE CTLR ASKED US OUR ALT AND WE REPLIED THAT WE WERE LEVEL AT 11000 FT AS ASSIGNED. THE CTLR ADVISED THAT WE HAD BEEN CLRED ONLY TO 10000 FT AND ASKED THAT WE DSND BACK TO THAT ALT; WHICH WE DID. THERE WERE NO APPARENT TFC CONFLICTS AS A RESULT OF THIS MISUNDERSTANDING. WE ADVISED THE CTLR THAT WE BOTH UNDERSTOOD THAT WE HAD BEEN CLRED TO 11000 FT; AND THAT WE HAD READ BACK THE CLRNC AND HAD NOT RECEIVED A CORRECTION. 2 QUESTIONS ARISE AS A RESULT OF THIS INCIDENT. ONE; HOW DID BOTH PLTS UNDERSTAND THAT THEY HAD BEEN CLRED TO 11000 FT; WHEN IN FACT; THEY HAD NOT. (IS IT POSSIBLE THAT THEY RESPONDED TO A CLRNC FOR ANOTHER ACFT?) AND TWO; WHEN THE COPLT READ BACK THE CLB CLRNC TO THE CTLR; WHY DID HE NOT ISSUE A CORRECTION? THE ANSWER TO EITHER QUESTION IS NOT READILY APPARENT.