Why am I cold when my sleeping bag is recommended for low temperatures?

Temperature sensitivity is subjective and difficult to quantify. It is dependent on many different factors (nutrition, training, hypersensitivity to cold, the state of your health, body size, climatic conditions, the thickness of the sleeping mat, etc.) Even in the controlled tests conducted in accordance with European Standard EN 13537 to determine the sleeping bag temperature ratings, it cannot be completely quantified.
An additional liner, warmer clothing (a cap or thick socks are particularly effective) or a sleeping mat with better insulation can make a difference.

Tip: choose a warmer model when purchasing a sleeping bag if you want to be on the safe side or know that you get cold easily.