No, Adams was pretty clear that each Zone represents a film exposure difference of one f-stop. So the difference between Zone V and Zone XIII is 3 full stops. As I suggested above I think you may making the error of interchanging scene luminance vs a printed result. But I can't tell for sure how you come up with 2.5 stops. This doesn't really prove anything. It just indicates that there is a certain relationship between the digital camera exposure reading and the specific way the camera translates to a digital image, most likely to either sRGB or Adobe RGB (1988)? This is somewhat akin to making a print. If a different color space were used for the outout, with a different tonal response, then the blinkies would likely happen at a different offset.