It is just bad luck. It may have tried dropping to 5dB and found instability or maybe it sees a medium length line (at 18dB attenuation) with a fairly low sync for the attenuation and knows it will not be worth trying!. At 16.5dB over 450m I have sync speeds up at mid 70s but know of one specific interferer that takes 6-8 Mbps.
M H C
taurus excreta cerebrum vincit