I've got an '08 SG with the Sundowner seat, and it doesn't look like yours at all. See the pics below. I think you got a factory reject, just take it to your dealer and show it to them, they should replace it for you.
I have a 07 King with the sundowner,mine fits like yours.I think it has something to do with 06 and 07 models.I just put a tank bra on and covered the gap up.
Thinking about it now,the seats may be designed now for 08's and up with the 6 gallon tank.If the tank were bigger the gap would not be there,who knows.but I have talked to other people with this combo having the same problem.
P/N 51630-09 fits 08 and later Touring with the 6 gal tank. P/N 51630-06 fits 97-07 FLHX and RK and other Touring with the long console P/N71288-03A. Maybe there was a mix up in numbers somehow? My 08 fits fine.
I just put the Sundowner solo seat on my 07 Street Glide and it fits fine. I agree with a couple of the other posts you might check the P & A catalog for the correct part number for your year bike.