Single CQI value can be mapped to multiple MCS value, say CQI 15 can be mapped to MCS 23 to 28. If UE reports CQI value 15 then it is ENB implementation which MCS to select among 23 to 28 (It is just an example). Why this variation is each MCS is having an different coding rate and efficiency of utilization. If you select 28 as MCS then efficiency is more but there can be chance of loss of more coding rate. If you slect 23 as MCS then throughput will be less because of less coding rate and efficiency .
it is ENB implementation how to handle the UE in relatime .