Your logic in the second paragraph does not compute. If I take your original 216 pts for 6 gallons, and dump a gallon of water in to the kettle, you end up with 216/7=1.031 SG (rounded up slightly). If you get your original points in the kettle for 6 gallons and then sparge to collect one more gallon, you will get more points into the kettle than 216. Sure the SG for the larger amount is lower, but you have more points in the kettle. That is what some of us are trying to say.
I see another post has the same point. Had to answer the wife's questions.
Read Kai's page, one of the best for this.