Question on WLS administration

[Giselle Dazzi]

The docs for the option "Low memory GC threshold" on Weblogic admin console say that this is the "Threshold level at which WLS will try to garbage collect once the granularity report has been met".
My question is: When is this condition met ? If at least once there has been a granularity report in the log ? Or if the server has been set to Warning state ?
The problem Im trying to address is that my "Low memory GC threshold" is set to 20% and my server has reached only 1% free memory and garbage collection did not run, I had to force it via console.
Thanks in advance, Giselle