The lower end thermostats (the "on/off" types) work this way. The range you've stated is perfectly acceptable for your hot spot - no need to be concerned as long as they stay around the 90 degree mark. The only thing to keep in mind is that sometimes they can get a bit warmer than your set temp simply because they are running at 100% power right up until they are told to stop. Just ensure you don't see any readings higher than 95 and you'll be good.

Also, you're doing the right thing by measuring the tank surface temp and not the substrate temp. As long as you don't keep the substrate too deep (which you shouldn't), your snake will have a nice comfortable home. You don't want to have to turn the heat up too much to reach proper temperature at the top of the substrate and risk burning your snake if they dig down to the tank surface.

The higher end thermostats (proportional types) work a bit differently. Instead of running at 0% or 100%, then provide just enough power to reach the temperature you have set and fluctuate slightly up or down as needed to keep it there.