No difference at all; the /1s were originally going to be ( half as many ) ten car units.
Then they changed to all five cars but the numbering was by then set in stone because the /6 was used for the c2c units.
Why would the C2C units being /6 effect the Greater Anglia units, surly numbering all the the GA units in the 720001 - 720133 series would have made more sense all round?