Michael's comments agree with shot peening theory. An internal 'ring' of compressed material is generated by shot peening. Some of the residual stress in this ring has to be relieved by increasing the diameter of the ring - generating a balancing tensile force in the underlying material. Hence the 'real' hole diameter increases. 'Apparent' diameter change depends on the induced change in surface roughness. This depends on the pre-peening roughness and the shot size.