请问这个的结果为啥是2呢？

#include "stdio.h"
struct packed_bit
{
unsigned a:2;
unsigned b:3;
unsigned c:4;
int i;
};
int main()
{
data.a=8;
data.b=2;
printf("%d\n",data.a+data.b);
return 0;
}

天上掉下来的data？