What physical meaning can these numbers have?
The EXIF standard doesn't actually have a single
dpi field, but it does have
YResolution fields, and these are where the dpi value displayed by most software comes from. The standard doesn't say specifically whether those values apply to the image source, e.g. a scanned document, or destination, e.g. a printed copy of the image, but since the entity that writes the metadata knows where the image comes from but can't know how it will be used, the only reasonable interpretation is that those fields relate to the image source.
The standard does say that if the value for the
YResolution tags is unknown, then
72 should be used. One could reasonably argue that this was a poor choice, since
72 is a valid value whose meaning shouldn't be overloaded. It probably would've made more sense to choose a value that couldn't possibly represent a real resolution, such as
-1, to mean "unknown." But that's not what the standard says, and so if your image has
72 for the resolution fields, you simply can't know whether the metadata writer knew what resolution to use or not.
The problem is compounded by the fact that some cameras set a value other than
72 when they can't really know what resolution the subject was recorded at, since that varies depending on how far away the camera was from he subject. Those camera manufacturers might have been trying to use the resolution fields to suggest a reasonable print resolution, but this only confuses the meaning of the resolution fields.
If you're trying to actually learn something about an image, you'll be better off using the
FocalPlaneYResolution fields, which record the sensor resolution at which the image was recorded. And of course the
ImageLength fields will tell you the dimensions of the image in pixels.