Thank you all for reply. You are right, 1 degC is represented as 274.15 internally.
BUT how the internal number is converted to degC? It shows that 274.150 (SGL) - 273.150 (SGL) could have an exact binary representation 1 (with exponent = 0 and mantissa = 1). For examples: 274.150 (SGL) - 273.150 (SGL) results 1, exponent = 0 mantissa = 1 275.150 (SGL) - 273.150 (SGL) results 2, exponent = 1 mantissa = 1 276.150 (SGL) - 273.150 (SGL) results 3, exponent = 1 mantissa = 1.5 .....