派生类的指针显示转化为基类的指针数据成员的值为什么发生了改变，求解释一下

最初的代码其中y等于2f，最终结果显示为为1.45f-45
#include
using namespace std;
struct Base1 { int x; };
struct Base2 { float y; };
struct Derived :Base1, Base2{ };
int main()
{
Derived* pd = new Derived;
pd->x = 1; pd->y = 2.0f;
void* pv = pd;
Base2* pb = static_cast<Base2*>(pv);
cout << pd->x << " " << pb->y << endl;
delete pb;
return 0;
}
而如果隐示转换就没有问题。
#include
using namespace std;
struct Base1 { int x; };
struct Base2 { float y; };
struct Derived :Base1, Base2{ };
int main()
{
Derived* pd = new Derived;
pd->x = 1; pd->y = 2.0f;
void* pv = pd;
Base2* pb = pd;
cout << pd->x << " " << pb->y << endl;
delete pb;
return 0;
}
不知道为什么😳