Hello Addy,
Thank you for your message.
How do you measure the horizontal accuracy? Do you use the checkpoints?
There should be no difference in absolute accuracy between DSM and orthomosaic.
I suggest you reach out to our support team, they might need to request an example dataset from you. Alternatively you can open a separate thread in PIX4Dmatic Questions/Troubleshooting - Pix4D Community, so we can discuss your issue there.
Hope it helps.
Kind regards,
Alexey
Update: I see that you opened a thread, let’s discuss further there