100 seems wrong, which is why i posted here.
The problem is that some software, when it reads the unit field “0: no unit, just pixel ratio”, assumes the units are actually dots per inch. So if the value is “1”, this is read as “1 dot per inch”, which can cause problems when printing the image, or compositing images, and so on.
So when software writes the jpeg, when there are no units (or units are unknown), it may be safer to write “100” rather than “1”.