Does it make sense for nitrates to drop at end of cycle?

I'm in about 3 weeks. Ammonia zero for about 4 days. Nitrite dropped to zero today test. I think it was very low yesterday. (I'm out at my kids hockey practice so I don't have my exact numbers on hand). I turned my skimmer on yesterday and it already sucked up some brown gunk. My nitrates read about 20 tonight. They were at least 40 4 days ago. I didn't expect them to drop without a water change. I'm still feeding some fish food daily.



I plan to pick up the last few items I need, order a CUC and change water before I get them. I'm not in a super rush so I will make sure my ammonia and nitrites are stable at zero. But is drop of nitrates normal?