I had this exact confusion when I first got my enclosure set up, but I realized that there are a couple things to remember. The gauge is determining the ambient humidity nearby, so when you spray, you probably get some on or right nearby the gauge. As the water falls away, it begins sensing the air around it again. As the dripping water then evaporates it disperses and raises the general ambient humidity. I make sure my humidity is up, but frankly I just make sure to mist and set up my dripper and make sure my cham is doing okay. Those gauges are really kinda p.o.s., and I use them more to get a general idea of moist/dry environment rather than an exact count.
Hope this helps!
Zack