结构体的一个奇怪现象求解

struct STRUCT_CMPP_CONNECT
{
    STRUCT_CMPPHEADER   stHeader;       
    char szClientID[6];                 
    char szAuthenticaltorClient[16];    
    BYTE iVersion;                     
    DWORD dwTimeStamp;                  
    BYTE iLoginMode;                
    STRUCT_CMPP_CONNECT()
    {
        ZeroMemory(this,sizeof(STRUCT_CMPP_CONNECT));
    }
};
struct STRUCT_WINPACKET
{
DWORD dwSequence_ID;
BYTE   bUsed;
time_t tSendingTime;
char Msg[402];
BYTE bTimes;
int MsgLength;
STRUCT_WINPACKET()
{
dwSequence_ID=-1;
bUsed=0;
time(&tSendingTime);
memset(Msg,0,402);
bTimes=0;
}
};
定义一个变量：
STRUCT_CMPP_CONNECT login;int len[6];
len[0]=sizeof(login.stHeader);
len[1]=sizeof(login.iVersion);
len[2]=sizeof(login.dwTimeStamp);
len[3]=sizeof(login.szAuthenticaltorClient);
len[4]=sizeof(login.szClientID);
len[5]=sizeof(login);得到的结果；
len[0]=12;
len[1]=1;
len[2]=4;
len[3]=16;
len[4]=6;
len[5]=40;
为什么len[5]不会等于len[0]+len[1]+len[2]+len[3]+len[4];