Ranco is a non-proportional thermostat, that will cut on and off, so the temps will vary.

What did you set it on? And what is the differencial?

This is from Reptile Basics thermostat FAQ

They turn on 100% when the temperature falls a degree or two below the set point and they stay on until the temperature gets a degree or two above the set point. This means that if you set your thermostat at 80 degrees your actual temps can vary between as low as 76-78 degrees and as high as 82-84 degrees.