As far as I understand it, solid state memory has a limited number of read/write cycles - less so than magneto-optical hard drive platters. Therefore the data is written (scattered) across the whole SSD to extend life, rather than reading/writing to the same sectors each time. The read speed is unaffected as it's solid state memory, so there's no mechanical read arm to move, like hard drives. Hence why defragging is good only for mechanical hard drives - the arm doesn't have to dance all over the platter.