Почему struct stFormat { unsigned int fmt;

unsigned int channelCount : 8;
unsigned int samplingRate : 24;
unsigned int blockCount;
unsigned short muteHeader;
unsigned short muteFooter;
};

sizeof(stFormat) == 16

но

#[repr(C, packed)]
struct Format {
fmt: U32<BE>,
channel_count: U32<BE>,
sampling_rate: U32<BE>,
block_count: U32<BE>,
mute_header: U16<BE>,
mute_footer: U16<BE>,
}

size_of::<Format>() == 20
?

https://docs.rs/ux/latest/ux/struct.u24.html через что-то такое можно поизвращаться