Seems straightforward to me, it depends on the time the Service Prosecuting Authority (SPA) was referring to. I suspect that the SPA was referring to the time of the offence, the Army Medical Board is referring to now.

While serving, like many soldiers, Sergeant Nightingale may have wanted to continue serving (there is nothing to indicate that he did not) and therefore may not have presented the full range of symptoms necessary to make a determination of being unfit to serve. His defence however relied heavily on aspects resulting from his injuries. Different times, perspectives and information presented.

I am slightly disconcerted by this case. A soldier was found with an illegal firearm and ammunition. I think the last time that someone was not jailed for this type of offence was a Dundee Grandmother With Her Grandfather's War Souvenir. The report on the link is worth reading as it goes into many of the legal issues. Firearms is a Reserved Issue with regards to Scottish legislation and there is no significant difference between Scottish and English law on this. English legal precedent is also applicable in Scotland.