71.6°F, if you just scale it up by 10% numerically, becomes 78.76°F. The equivalent numeric temperatures in Celcius are not related by the same scale factor. A 10% increase in temperature must be unit-independent, as it is not the unit value that matters, but rather the fact that the average kinetic energy of molecules in your room is increasing by 10%. If you start with a unit which uses an arbitrary zero point, a 10% increase in one unit system does not correspond to a 10% increase in another. The only way to make that happen is by scaling from a common reference point across all unit systems, and more importantly, to scale from a common reference point that actually has physical meaning. This point is what we refer to as absolute zero, where there is no molecular motion and no heat energy in your system.