26mm is the 35mm equivalent length; essentially it’s a standard figure to make numbers comparable across cameras/phones. Good explanation here.
4.25mm is the actual focal length of the lens
If you had a 35mm camera, a 26mm lens would give you the same field of view as the lens on the iPhone.
It’s a convention to talk about phone camera focal lengths in terms of their 35mm equivalents
Thanks for the explanation.