For an explanation you may want to inspect "Recommendations for Biochemical Equilibrium Data".Recommendations for Biochemical Equilibrium Data 1, which states:
Buffer and pH. If only a limited number of measurements are to be made, they should be carried out at pH = 7.0 and, if possible, also at a pH value at which the apparent equilibrium constant $K_c^\prime$, has little or no dependence on pH. ($K_c^\prime$ is defined in a later section.) If direct measurements at pH = 7.0 are not practicable, the calculated values for this pH should be reported. The procedure used in making these calculations must be carefully described. Care should be taken that the solution is adequately buffered so that the pH is well defined throughout the experiment. It is desirable to determine the effect of varying the nature and concentration of the buffer in order to identify buffer effects. Buffers that are known to interact with reactants (including macromolecules) or salts, such as phosphate or pyrophosphate in the presence of divalent metal ions, should be avoided.
The highlighted portion means that reported values for biochemical reactions are (or should be) referenced to pH 7.0. The data in Table 3.6 refer to this biochemical standard state.
Another question on the subject of equilibrium constants also addresses the importance of properly considering reference states.
Reference
1 The Journal of Biological Chemistry (1976), Vol. 261, No. 22, pp. 6859-6885.