struct with members' sizes

I'm using C51 v3.20, and when I compiled the following structure, the compiler allocates 6 bytes for it.

typedef struct t_channel_info {
	int target_temp:9;
	unsigned char control_mode:2;
	unsigned char status:2;
	unsigned char warming:1;
	int current_temp:9;
	unsigned char warm_proportion;
} t_channel_info;

Should not it allocate only (9+2+2+1+9+8)/8=4 bytes?

But, when I partition a 9-bit integer to 8 and 1 bits chars, the size reduces to 5 bytes.

Is there an explanation?