IIRC co-terminal standby is only offered if the same fare basis and price is available.
examples
If both SEA-SFO and SEA-OAK have Qa7n fares for 79.00 OW --->Standby ^
If SEA-SFO both have Qa7n fares but SFO is $119 and OAK is $79 ---> Standby ^
BUT you have to pay the fare difference.
If SEA-SFO is a Va14N for $119 and SEA-OAK is Qa7n for $79 ---> Standby
... you would have to pay the change fee and any fare difference up to applicable walkup fare to SFO
If I'm not recalling correctly then I'll let you know Monday
