What will be the temperature on Earth when Sun finishes its main sequence?
The difference is that your analysis is assuming that the albedo stays fixed, so the surface temperature simply scales like luminosity to the 1/4 power.  The Wiki entry is including feedback from the greenhouse effect, which will tend to further increase the surface temperature.  Note that an analysis that just looks at solar irradiation would get way too low a surface temperature for Venus, for example.  I can't speak to the accuracy of the feedback included-- as I understand it, that is far from a simple effect to include.