The algae may be releasing exudates which could be picked up by the total ammonia test, but not the badge, maybe. Amino acids specifically. These exudates can fuel heterotrophic growth which may also play a role.Algae/other items. Plus - If it continues that the unlit rock does not keep up with the lit rock - it also goes against the hypothesis of some that nitrification bacteria cover every surface equally - OR - at least the other photosynthetic organisms on 'lit rock' do more.
BTW - To my view - and will see later today - the Seachem Alert (which is supposedly measuring 'free NH3' - has started to turn lighter (more yellow) than the API total Nitrogen tests suggest. PS - From now on - I will also be testing Nitrates.