This is the frustrating part... It will fluctuate for a little. This is why generally the recommendation at the end of the cycle is to wait for a full week (at least 5 days) of solid zeros before adding the fish, whenever possible. There are a lot of factors working together in the tank, and things like this can happen. Why? I don't know, but they do. Perhaps the bacteria are building up a biofilm rather than processing ammonia/nitrite. Perhaps they are multiplying more and processing less. I don't know, but I do know that these "set-backs" are fairly common - that's why you focus on the overall trend rather than the individual data point. You are still processing ammonia and nitrite at a good level. Did the pH drop at all? That's certainly a potential reason for the bacteria to slow their process down a bit.