Digging a little deeper...

  (0 degrees Celsius) + (0 degrees Celsius) - (0 degrees Celsius) = 0 degrees Celsius

  (0 degrees Celsius) - (0 degrees Celsius) = 0 kelvin

  (0 degrees Celsius) + (0 kelvin) = 0 degrees Celsius

The last 2 of the 3 are wrong, so I'd say this is definitley a bug.

EDIT: I think I figured it out. It's converting everything after the first temperature to Kelvin, without converting the first temp to Kelvin, and then just doing the arithmetic on the numbers...

  0C + 0C => 0C + 273K => 0 + 273 = 273

  0C + 0C - 0C => 0 + 273 - 273 = 0

  0C - 0C => 0 - 273 = -273 = 0K (ok, weird this one then converts back to K)

  0C + 0K => 0 + 0 = 0

EDIT 2: While this does match the results seen by google, I think _delirium's solution is more likely the reason.