A weird Timinig issue with “clEnqueueNDRangeKernel

I'm new to opencl and I'm experiencing a weird issue with it! I have a reduction kernel and I repeat it several times! The problem is that when I profile the execution of kernel the elapsed time (queued->end) is almost same and a bit increasing but when I measure the elasped time within "C++" code the time for the execution of line "clEnqueueNDRangeKernel" increases with a rapid rate!! I have attached both the code and the output of profiling! :shock:

// execute the kernel
globalWorkSize[0] = this->reduction_NumBlocks * this->reduction_NumThreads;
localWorkSize[0] = this->reduction_NumThreads;

//Start Time
ttt.start();

clErrNum = clEnqueueNDRangeKernel(clCommandQueue, kernelReduction, 1, 0,
globalWorkSize, localWorkSize, 0, NULL, &timing_event);
// check if kernel execution generated an error
oclCheckError(clErrNum, CL_SUCCESS);

clFinish(clCommandQueue);
ttt.stop();

//Check Elapsed Time
clGetEventProfilingInfo(timing_event, CL_PROFILING_COMMAND_QUEUED,
sizeof(time_start), &time_start, NULL);
clGetEventProfilingInfo(timing_event, CL_PROFILING_COMMAND_END,
sizeof(time_end), &time_end, NULL);
cout<<"ElapseTime(Execute):"<<(time_end - time_start)/1000<<"us\tTTT:"    <<ttt.getElapsedTimeInMicroSec()<<endl;

and this is the output:

    GeForce GTX 550 Ti
    Device Timer Resolution:1000ns
    GpuExecutionTime:160us   C++ElapsedTime:177
    GpuExecutionTime:156us   C++ElapsedTime:167
    GpuExecutionTime:156us   C++ElapsedTime:166
    GpuExecutionTime:189us   C++ElapsedTime:242
    GpuExecutionTime:158us   C++ElapsedTime:215
    ...
    GpuExecutionTime:156us   C++ElapsedTime:253
    GpuExecutionTime:162us   C++ElapsedTime:261
    GpuExecutionTime:157us   C++ElapsedTime:262
    GpuExecutionTime:156us   C++ElapsedTime:254
    GpuExecutionTime:157us   C++ElapsedTime:254
    GpuExecutionTime:160us   C++ElapsedTime:261
    GpuExecutionTime:167us   C++ElapsedTime:279
    GpuExecutionTime:157us   C++ElapsedTime:264
    ...
    GpuExecutionTime:159us   C++ElapsedTime:263
    GpuExecutionTime:157us   C++ElapsedTime:261
    GpuExecutionTime:157us   C++ElapsedTime:260
    GpuExecutionTime:157us   C++ElapsedTime:263
    GpuExecutionTime:264us   C++ElapsedTime:384
    ...
    GpuExecutionTime:156us   C++ElapsedTime:304
    GpuExecutionTime:161us   C++ElapsedTime:314
    GpuExecutionTime:157us   C++ElapsedTime:308
    GpuExecutionTime:160us   C++ElapsedTime:305
    GpuExecutionTime:158us   C++ElapsedTime:311
    GpuExecutionTime:156us   C++ElapsedTime:308
    GpuExecutionTime:157us   C++ElapsedTime:312
    ...
    GpuExecutionTime:157us   C++ElapsedTime:326
    GpuExecutionTime:158us   C++ElapsedTime:326
    GpuExecutionTime:159us   C++ElapsedTime:330
    GpuExecutionTime:158us   C++ElapsedTime:328
    GpuExecutionTime:158us   C++ElapsedTime:335

Any kind of help is appreciated.

P.S. The size of input and other related vairables are fixed!