Cycling new tank - ammonia dropped off once (so far)

[rojees]

Ok so here goes:

its about 16 days since I started my cycle, as in adding ammonia solution.
Levels have been constant around 2ppm, and I have added Microbacter 7 as well.

Now I am not using rock, instead I am using 2 Mantis Bio filter blocks.
Tank is also bare bottom, so I get the cycle will take longer and all that. Prob not the way to go long term but this is how it will be setup for now.

2 days ago, i.e. about day 14, the ammonia dropped to about 0.5, so I dosed it back up to 2ppm. I used the API strips and the nitrites came back to around 0 (very light coloured).

The ammonia has not since lowered (I know its only 2 days after) and I tested for nitrites again using the API strip, again I would assume around 0, colour was very light. I thought maybe the strips were faulty? So I went off to get a Sera Nitrite test, and it showed 0.5 reading.

My question, why did the ammonia drop off on day 14, and nothing since then?