What they don't say in the info is how each contributor's info to FlightRadar24 can be different. The receivers come with a small antenna designed for indoors use that you position near a window. This will give limited coverage sometimes - for example my box is on the south side of the house and my coverage to the north is much lower than the coverage to the south. Some of the contributors will have roof mounted antenna which will give up to 300 miles coverage.
Don't forget that not all the contributors operate their receivers 24hrs a day, they may only be contributing when they have their PC switched on.