Why cudaMemGetInfo total memory less than nvmlDeviceGetMemoryInfo total memory?

Accelerated Computing CUDA CUDA Programming and Performance
1

I found cudaMemGetInfo total memory is less than nvmlDeviceGetMemoryInfo total memory. I want to know why and what does 24152899584-23661248512=491651072=468.875MB means.

#include <iostream>
#include <nvml.h>
using namespace std;

int main() {

    // 使用cudaMemGetInfo获取显存信息
    size_t cuda_free = 0, cuda_total = 0;
    cudaSetDevice(0);
    cudaMemGetInfo(&cuda_free, &cuda_total);
    cout << "cuda total:" << cuda_total << endl;
    cout << "cuda free:" << cuda_free << endl;
    cout << "cuda used:" << (cuda_total - cuda_free) << endl;

    nvmlInit();
    nvmlMemory_t mem;
    nvmlDevice_t device;
    nvmlDeviceGetHandleByIndex(0, &device);
    
    nvmlDeviceGetMemoryInfo(device, &mem);
    cout << "total:" << mem.total << endl;
    cout << "free:" << mem.free << endl;
    cout << "used:" << mem.used << endl;
    nvmlShutdown();
    return 0;
}

result: cudaMemGetInfo total memory is less than nvmlDeviceGetMemoryInfo total memory. And nvmlDeviceGetMemoryInfo total memory is equal to nvidia-smi result.

cuda total:23661248512
cuda free:23459921920
cuda used:201326592
total:24152899584
free:23459921920
used:692977664