Can I not travel the full length of a journey as my train ticket says?

Using the Southeastern app, it is possible to get a ticket from Dartford to Hackney Wick via Gravesend, HS1 and Stratford Intl/Stratford for £7.00 off-peak. However, a ticket from Dartford to Stratford international via HS1 costs £22.10. Am I allowed to buy the ticket to Hackney Wick, get off at Stratford International and not ride the extra stint on the Overground? Thanks.