[tor-relays] wild swings in Advertised Bandwidth

Hi, I'm running a low-volume bridge on a raspberry pi. Globe.torproject says that my fingerprint is 1BCD3EBEFE17EEB86EEDE21D5E2DB8468E2864CF . I'm keeping an eye on myself and traffic via globe.torproject and my advertised bandwidth makes wild swings: one day its 56 kB/s and the next day it's 8 kB/s. When I read that "The minimum BandwidthRate setting is 20 kilobytes per second", I am confronted with the fact that 8 is less than 20 and it doesn't seem worth it keeping my bridge up and working. Why would a bridge make such swings in advertised bandwidth? And is there any good reason to keep the bridge up and working if it sinks to 8 kB/s? Thanks, J Chase