I see it as similar to buying a tent. I could never figure out how they arrived at their capacity claims.
Then I found out that the first two people were counted as adults, and the remainder children.
So an 8 man tent was sized to sleep 2 adults and 6 kids.
Not as sure how the spa industry does it. In our case, the Optima is considered a 7 to 8 person spa. And you can actually seat 8 adults, one in each corner, and one between each corner. The mid seats don't have much in the way of jets, but at least you can sit and soak.
Brewman