You overlooked some weight to arrive at 13%. It actually works out a bit worse.
The 11500 is the axle weight of the trailer without the bike loaded. The 1700 lb pin weight is with the bike.
After loading the bike the pin weight will be 1700 lbs: The trailer axle weight will be 11500 + 900 + 700 = 13100 so the tital trailer weight without water and groceries will be 14800. 1700/14800 is about 11.5%.
On the other hand the pin weight of 11.5% of the total trailer weight describes how the weight is distributed. The fact that there is a 900 lb bike on the back is not additional information.