The thermostat is functioning fine, it's your your application ( the temperature sensing tube in direct contact with the evaporator coils) would call for a lower temp range thermostat.

The range your current thermostats'  would be correct for an Air-Temperature sensing application, such as an air-circulating soda machine, (like a Vendo V63) ,

but your application is a direct contact type ( direct contact with the evaporator) which runs at about 7-12 degrees lower than the surrounding air temp.  so you'd want a lower temp range unit. here's a link to Grainger, I'd use a RANCO model no. A12-700

http://www.grainger.com/Grainger/items/3XL05