Not comparing like with like
A "true" news story generally gets reported by many different channels concurrently. A made up one has a single authoritative source.
To compare the two on Twitter, you'd need to aggregate all the "primary" sources of the real news story, vs following the propagation of just one tweet for the made-up story.
To put it another way: a BBC tweet may not get many retweets, not because no-one's interested in the story, but because they prefer the reporting in the Grauniad or Le Monde or the Chicago Tribune instead.