Sometimes using google calculator is so much fun, you get very interesting results.

(1 degree Celsius) + (1 degree Celsius) = 275.15 degrees Celsius

I add Fahrenheit’s and answer comes up in celsius
(1 degree Fahrenheit) + (1 degree Fahrenheit) = 238.705556 degrees Celsius

Giving it a thought one can come up with the reason why google gives you that answer. Kelvin is the unit increment of temperature, hence its converting celsius into kelvin and then adding the numbers, then converting them back into celsius.

[K] = [°C] + 273.15
So
1°C + 1°C = 548.3 K = 275.15 °C

Similarly
[K] = ([°F] + 459.67) × 5⁄9

1°F + 1°F = 511.856K = 238.7056 °C

Now that we know why the answers come up the way they do.
Question remains why 1 + 1 is not 2 😉