Are bandwidth authorities concentrating too much bandwidth in one area?
Are there too many bandwidth scanners and servers in the same area (western Europe / eastern North America) ?
Are the bandwidth scanners and servers too close together? (If they are in the same data center, then any 2 relays that are in the same data center will get unrealistic results.)
There could also be specific bugs in sbws (legacy/trac#33350 (moved)) or torflow (unmaintained).