Just spoke with Honeywell.

Apparently, the random 35% running time of the fan is based on a
particular schedule. For instance, if heating and/or cooling has been
set for the wake period, and then fan has been set to run with the
system, the fan will be on 100% of the time while either heating or
cooling is on. If during the wake period, the heating or cooling stops
AND the conditioning period is less than 35% of total wake period, then
the fan will run for up to 35% of the wake period.

This cannot be that accurate as it will require some prediction on the
thermostat's part. But, I guess it can help to circulate air when the
conditining part is off.

Hope this helps someone out there.