It’s purely operational - 8 car Thameslink 700s fit into the south end bays at Cambridge the new GA units don’t do so have to run through to Cambridge North
Yes, correct.
Though I reckon than doing one service half-hourly from North rather than two individual hourly services might be better at stimulating more "turn up and go" type demand overall, particularly with cheaper GA fares to Liverpool Street (presently only the slow GA service reaching North, so not that attractive at the moment).