What is the true "normal"?

As I understand it, the "normal" focal length is the one that delivers the same perspective as you would get with the naked eye.

A rule of thumb often quoted is that this is achieved when the focal length of the lens is the same as the diagonal of the sensor. For my K-7 at 23.4x15.6 mm this would be almost precisely 28mm.

I threw my zoom lens on the camera the other day and focused on something at a reasonable distance, then adjusted the FL until the image magnification was about the same in my right eye through the viewfinder as my left eye without it. I was even able to get stereo vision by looking through both eyes at the same time. The surprise was that this occurred at about 55mm!

I know that the viewfinder shrinks the image a little (0.95 magnification) but I wouldn't expect it to make this much difference.

Could it be because of the glasses I wear for short-sightedness?

What are your experiences?