Ranco thermostats are non-proportional. Meaning, when the circuit is closed, it has full power to the flex watt. So likely, if the difference you are seeing is a couple degrees higher, it's probably due to the flex watt still being on it's warm up when the Ranco shuts it down. The probe is on the inside of the cage, or should be, and the flex watt should be outside. Don't forget the cage material it'self, it's still on the warm up too, and likely to get a couple degrees higher then your settings, then it cools off and the circuit closes again to reheat. A proportional thermostat, like HerpStat and Helix, adjusts the power to the flex watt, dimming or brightening if you will, to maintain your settings a little closer and more constant as well, rather then the fluctuation you'll see with the Ranco.
If you are going to run more then one cage off the strip, you have to use all the same heating materials (ex. 2 ft Flexwatt each, or what ever) same lengths of cord to each 9so they all draw the same amount of current) to keep them all within the same temp ranges, since you only hae one probe. In addition, you need to turn it all off until it's all room temp, then start them all at the same time. But don't be surprised if you get a stubborn unit that gets warmer or cooler using this route.
The above is my personal experience, opinions may vary, or someone may know something I don't. Keep that in mind.
What are you checking the thermostat readout with? If you don't have a temp gun, I highly suggest one. I found out a couple of my Rancos were a few degrees off what the read out said. Never would have known had I not checked them. I also use an addition thermometer to independently check everything.
Hope that helps.