Many articles nowadays are behind a paywall. So when I want to add an article from a paid newspaper to Pocket, I figured out that that portal just tries to grab the article itself instead of taking it from Firefox directly. Which means that you just see the part of the article you see when you aren’t logged in.
Does that make any sense? Not really.
Other read later clients take the loaded article working with articles with payalls. I know Flyleaf does this, although it is Apple only, so you’ll need to research for other platforms but they’re out there.