I'd almost have to say something is coming off the rock to screw with the test. Or.. What did you use to cycle the tank? Probably the more important question.
I've only used API for nitrites and to be honest I not only find it useless to measure but I've never seen anything tracable even when starting a new tank. With nitrates being the end result, and you being at 64 I myself would let that number come down a bit before adding something. Adding live stock will make that number worse, which could lead to problems without having the proper bacteria growing on the rocks yet.
I am about 2 weeks into my cycle, I finally got 0 ammonia, just waiting for nitrites, at about 5ppm to come down. In theory, once nitrites hit 0, I am cycled. I do a large water change, add my clowns back in, and pick up some cheapy zoanthids, and away I go.