The best explanation for the use of the value 273 K for temperature 2 is: Temperature 2 was 0.00 °C so we add 273 to convert it to Kelvin.
This is because the Kelvin scale starts at absolute zero, which is equivalent to -273.15 °C. Hence, to convert from Celsius to Kelvin, you add 273.15 (often approximated as 273 for simplicity) to the Celsius temperature.