The B1HT manual doesn't specify what the temperature range is by default (it may not have one). Usually thermostats have a built in range that sits around your setting. One main reason for this is so that it isn't constantly turning your heat source on and off which could damage your light/heatmat/etc. The thermostat I have on my chest freezer (beer cooler) is set to keep it within a range of +/- 3 degrees (adjustable) so that it isn't constantly turning the compressor on and off which could damage it.
If you check the accuracy of your digital thermometer, you could tweak the setting on your thermostat to compensate.