Nartia Express Ticket on Eki Net – Is it really that inexpensive?

I am looking at purchasing a Narita Express ticket on EkiNet for Narita Airport Terminal 1 to Tokyo Station. Looking 2 weeks in advance. The price seems to be only 1120 yen for a ticketless discounted ticket compared to 3070 yen for Narita Express purchased same day at the airport. Is there something I am missing? What happens if I miss the train due to it taking longer than expected at immigration? It just looks too good to be true. The example shown here is for Narita Express No 2. https://imgur.com/a/Ajh4FCE