Fewer than half were already on the register. The majority were not. This is somewhat exceptional, I would say. Typically, fewer people register during the writ period. It explains, perhaps, the large number of unreturned ballots—because people received them so late that they didn't dare return them—or late returned ballots.
The fact that it was not a fixed-date election and that it was a high-turnout election, so people were interested in participating, resulted, presumably, in the late demand for international ballots.