Resolution, defined as ppi, is a print parameter. It's only metadata, not a basic file property. It's irrelevant for web/on-screen viewing (unless you intend to print them later).
Think about it: pixels per inch. The file itself is just pixels, but ppi defines how big each pixel is on paper, and so defines physical size.
On screen, the image pixels align with the screen pixels (at 1:1 view). IOW it is displayed at the monitor resolution, somewhere between 90 and 110 ppi. No matter what the original ppi is given as.
A lot of web apps simply strip the resolution metadata from the file (Photoshop's Save For Web does that for instance) - because it just doesn't matter. But when you open that file somewhere else, a default ppi is often assigned because for printing purposes it has to assume something.
Where the 240 figure comes from I don't know, except that it's the default in ACR/Lightroom.