According to the manual, I can set my RX10 iv to auto ISO and chosing the standard setting it will set the shutter speed relative to the focal length of the lens and then adjust ISO for a perfect exposure.  I cannot however get this to work.

As an example I have tried the following all at f4   

Focal length                Shutter speed

24mm                           1/250

100mm                         1/125

300mm                         1/250

500mm                         1/250

600mm                         1/250

I would have expected the shutter speed to be the reciprocal of the focal lenght.

Anybody have any ideas?