why 32bits RGB image, not 24bits RGB

Hello,

I found in ImageJ, if you open a RGB image, if click show info, you see 
that it is 32bits per pixel. As I understand, RGB image should be 24 bits 
RGB instead of 32 bits, since each channel is 8 bits. In ImageJ, it 
calculate the size of a RGB image based on 32 bits per pixel. For example, 
one RGB (1024x1024) image I used shows size 4MB. But if I save it to a 
disk, the file size is 3MB which is calculated based on 24 bits per pixel.
Why in ImageJ, it shows 32bis per pixel for RGB image? Is that a mistake or 
ImageJ has special data structure for RGB image?
Thanks