Probably the manufacturer only certifies down to a temperature that is required for what they expect sales for.
The ATR-42 had -40°C as its lowest OAT for takeoff(this is what I found on an internet search anyways) but I flew ones that had a lower temperature certified down to -54°C as it was operated in colder conditions. What exactly had to be done to achieve this? I'm not sure but I think our company paid the expenses to certify the lower OAT as they were purchased used.