Slight Humidity Drop??? WHY?!

Is it normal to have the humidity drop a bit during piping? I mean my humidity is still good, 81%, but I just noticed that it wants to fluctuate a bit. It's well up there, but it's kind of making me nervous because of all I've read about even a 1/2 percent humidity drop can't be good. Or maybe it was always that way, but normally it's been ranging 85%, I did add a bit of water this morning when I noticed, I didn't open it up, but poored a bit into the air holes of the hova bator. So, I'm just curious and trying to remember if it did the same thing with the last batch that piped. I know while they hatched and the moisture of the wet duckling made it rise to almost 90%. So, just keeping an eye on it. Temp seems to fluctuate a tad to, and I haven't done anything to disturb them.