Default value of max dynamic shared memory

Accelerated Computing CUDA CUDA Programming and Performance
1

I’m working on CUDA 11.2 and Compute Capability 8.6. I have a kernel which might require more than 48KB of dynamic shared memory, so I wrote

int dynamic_smem_size = // ... some calculation logic
if (dynamic_smem_size >= (48 << 10)) {
    cudaError_t result = cudaFuncSetAttribute(
        MyKernel, cudaFuncAttributeMaxDynamicSharedMemorySize,
        dynamic_smem_size);
}
// launch the kernel..

but launching the kernel gives me cudaErrorInvalidValue when the dynamic_smem_size is 48964. Then I tried

  cudaFuncAttributes attrs;
  cudaFuncGetAttributes(&attrs, MyKernel);
  LOG(INFO) << attrs.maxDynamicSharedSizeBytes;

the result is 48724. It looks like the the default value for cudaFuncAttributeMaxDynamicSharedMemorySize for my kernel is not exactly 48 << 10 bytes. Why is that?