Difference in calculated pH and the real pH of a phosphate buffer?
The flaw in your calculations is the assumption that there exists a pK that is a constant independent of concentration.  

In reality, pK is a function of ionic strength.  

At infinite dilution (zero ionic strength) the pK is 7.199 as you assumed.

At 0.1M phosphate concentration, the pK is 6.81.

See Phosphate Buffers which cites to A. A. Green, "The Preparation of Acetate and Phosphate Buffer Solutions of Known PH and Ionic Strength", J. Am. Chem. Soc. 55, 2331, (1933).