Septum deviated to right but I feel airflow is restricted in right nostril - what's the reason for this?

It might sound contraintuitive, but many patients experience obstruction to flow in the non-deviated side. This relates to the fact that your turbinates will increase in size if allowed to grow, and a deviated septum away from the turbinate (left in your case) could potentially lead to overgrowth (hypertrophy) of your left turbinates which subsequently could restrict airflow. The other reason is that a septum could be deviated to one side in the front but start off crooked from the opposite side in the back, almost like a swinging door at the hotel lobby - the opposite sides would be equally angled.

Hi and welcome to our forum!
Without an internal nasal examination, I am not able to adequately answer your question. A sigmoid shaped septal displacement (like the letter "S") may obstruct one or both nostrils. The anterior segment may be deflected in one direction while the posterior segment may be deflected in the opposite direction. Septoplasty will straighten the septum at midline to eliminate nasal airway obstruction. In some instances, partial removal of the turbinates may be required as well.
Best wishes...