Those are not strictly internet monitors; they are network traffic monitors. Since you are using a hub rather that a router they may be measuring every bit of traffic on the network between all of the pcs rather than just the traffic of each pc.
At a guess, do you have 4 pcs (4x5 MB = 20 MB)?
Does the monitor on each pc show roughly the same amount of traffic?
Can you analyse the traffic by source and destination?