Further to the comments about filtering, the picture below shows part of the screen from my SBS-1e. These are the contacts picked up by my hardware. But note that only those which I have highlighted with a white box actually show up on the "radar screen".
So Dash-8, Fokker 50, Embraer 170/190/145, CRJ7, USAF, many bizjets, early 737s, etc only send altitude information - no position data which can be plotted. That means they somewhere in range but a 100 mile radius gives a lot of options.
Finally Qatar A330 A7-HJJ appeared yesterday on the SBS-1 over the Bristol Channel so I thought I'd check FR24. It wasn't showing so real-time filtering is happening.
Ian