I also use Reptibark, it seems to hold the humidity well. When I change it the humidity is high (about 50-60%, up to 65%) for about a week or so then it starts dropping, but it always evens out at around 40-50%. I've seen it go as low as 35% when we had the A.C. on and outside humidity was low (doesn't happen much in Northern VA).
Still I would say don't sweat it, just keep an eye on it. I don't worry about it but I do read it just in case. Just like the temp, I know they're going to be the same when I check it everyday, but I still check.