An ambient temperature of around 41 °F is normal in modern refrigerators that are compartmentalized. Compartments for meat are closer to 32 °F, and compartments for vegetables are closer to 50 °F. I believe the "40 °F or bellow" guideline given by the USDA Food Safety and Inspection Service is based on generic non-compartmentalized refrigerators. If you're a vegetarian without separate temperature-controlled compartments, 41 °F would be satisfactory.

This is simply about time and the microbial race. If you consume everything in your refrigerator within a couple hours of bringing it home from market, even higher temperatures could be safe. The point is that food becomes less safe with higher temperatures and longer storage duration. Incidentally, the math behind calculating the proper amount of yeast for a dough formula given a specific temperature is applicable to determining how long food remains safe at a specific temperature relative to its type. Without the math, and without knowing more of the specific refrigerator, "40 °F or bellow" is definitely the guideline to use.

- red.november