ZBW CTLR MISSES INCORRECT READBACK FROM DC9 FLC WHICH CONFLICTS WITH CARJ.

Narrative

THE OPERROR WAS THE RESULT OF A PLT READING BACK A WRONG ALT ASSIGNMENT AND MY FAILURE TO CATCH THE READBACK ERROR. THE DC9 (ACFT X) WAS AT FL280 COMPLAINING OF LIGHT TO MODERATE CHOP. I TOLD HIM THAT I HAD RECEIVED COMPLAINTS AT FL260 AND FL280 AND THAT FL240 WAS SMOOTH. HE REQUESTED FL240. I TOLD HIM IT WAS UNAVAILABLE AND TO DSND TO FL260. HE WENT TO FL260 AND WAS STILL GETTING CHOP SO I TOLD HIM TO DSND TO FL250 AND HE READ BACK FL240; WHICH I MISSED. I CALLED OUT TFC TO HIM AT 2 O'CLOCK LEVEL AT FL240 AND HE SAID 'ROGER;' AND CONTINUED TO DSND TO FL240. I NOTICED HIS MODE C READING FL247 AS HE PASSED THE CARJ (ACFT Y) AND ASKED HIM HIS ALT. HE SAID FL246 DSNDING AND I ASSIGNED HIM FL250 AGAIN. THE WX WAS A FACTOR IN THE ERROR BECAUSE OF THE CHOP. I HAD 4 ACFT LEVEL AT FL240 BECAUSE IT WAS SMOOTH. THE PLT OF THE DC9 EXPECTED TO GET FL240 BECAUSE OF THE SMOOTH AIR AND I'M SURE THAT WAS WHAT WAS ON HIS MIND. I STILL SHOULD HAVE CAUGHT THE READBACK ERROR; BUT WHAT I DON'T UNDERSTAND IS WHY DIDN'T THE PLT QUESTION ME WHEN I TOLD HIM ABOUT XING TFC AT FL240?