same exact issue here!!! I just posted a long post in my 20gal journal about some weird parameter stuff going on in my tank (the more recent issues are my own doing, but the ammonia thing also stumped me, but I can at least share the experience!).
What I think happened in my case was two things: 1) chloramine in water (this is tricky, because the API test kit may or may not measure the ammonia that comes from tap water treated with Prime. I've both measured and not measured ammonia directly after a water change, so not sure there). 2) rotting stuff in filter/plant debris/rotting plant parts. Mine was really obvious since my plants got a little too cold for comfort in shipping, but I don't think it takes much in an uncycled tank. Another tricky thing for me, is that I STILL have bits of dead leaves that are stuck to the original plants they died on, and they don't just vacuum off. So I'm sure they're rotting and producing ammonia, but they're hard to get rid of since if I take the vac too close to the plant, the whole stem/leaf gets sucked in dislodging the plant. I suppose I could sit there and try to hand-pick off all the slime, but at the end of the day it won't really net me anything other than aesthetics I don't think.
On the plus side: consider this an automatic cycle doing its thing!!! My ammonia went away around week 3, and now I'm fighting the nitrite battle. I would have thought the "planted cycle" thing would have worked better for me, but I probably did many things wrong and was impatient, so it didn't. shame on me I guess, but here we are.
At least I have a lot of good "FAQ" entries to put into the planted cycle article that I'm working on!!
It's nice to know that I'm not the only one