I think it is the fact that your NVR is an "/A" model which is causing the firmware to be rejected.
It seems to be a common issue. If you look at this thread, other users have found that the /A models do not support the same firmware as the non-/A models.
So, it's likely that the version you have installed is the last one that was released for your model.
If you look at the attached table, you will see that there are no E2/8P/A models shown under the E-series firmware at all.