为何我用不均匀硬币产生出来的均匀硬币的比例是7:3左右，即使用了srand也只能到6：4，

悬赏园豆：5 [待解决问题]

代码如下：

#include "stdio.h"
#include "stdlib.h"
#include "time.h"
#include "windows.h"


int unbalancedcoin(){
    
    int a = rand() % 10;
    if (a > 6)return 0;
    else return 1;
}
int banlancedcoin(){
    
    SYSTEMTIME currentTime;
    GetSystemTime(&currentTime);
    srand(currentTime.wMilliseconds);

    int a = unbalancedcoin();
    int b = unbalancedcoin();
    while (a == b){
        b = unbalancedcoin();
    }
    return b;
}
int rand6(){
    int a = ~(1 << 31);    
    while (a > 6){
        a = 4 * banlancedcoin() + 2 * banlancedcoin() + banlancedcoin() - 6;
    }
    return a;
}
void main(){
    
    int sum[6] = { 0 };
    int tmp;
    int a[10000];
    /*for (int i = 0; i < 10000; i++){
        tmp = rand6();
        a[i] = tmp;
        sum[tmp - 1]++;
    }
    for (int i = 0; i < 10000; i++){
        printf("%d\t", a[i]);
    }
    printf("***********\n");
    for (int i = 0; i < 6; i++){
        printf("%d\t", sum[i]);
    }*/
    
    for (int i = 0; i < 10000; i++){
        
        tmp = banlancedcoin();
        a[i] = tmp;
        sum[tmp]++;
    }
    for (int i = 0; i < 10000; i++){
        printf("%d\t", a[i]);
    }
    printf("***********\n");
    for (int i = 0; i < 2; i++){
        printf("%d\t", sum[i]);
    }

    getchar();
}