sizeof of struct with bit Field

please explain me why the sizeof (C51) returns 6 bytes for this struct instead of 5:

typedef struct DEKO_COMM_HEADER
{ UINT16 m_uiMsgOpcode;
UCHAR8 m_sblMsgPriority :2;
UINT16 m_sblDataLength :11;
UINT16 m_sblMsgTimeStamp :11;
}DEKO_COMM_HEADER, *PDEKO_COMM_HEADER;

UINT16 is typedef for unsigned int
UCHAR8 is typedef for unsigned char

thanks