I don't know for sure what you're laughing at, but if you thought I was saying a 9-footer has a playing surface 108" long and a standard 7-footer has a playing surface 84" long, no, no, no, of course not.
The 7, 8, and 9-foot labels can be thought of as "nominal" (not actual) lengths computed as 2 x (playing surface width plus width of two cushions). So 2 x (50 + 2 + 2) = the "nominal" 9-foot length.
A second way to think of it (as you sort of said) is to think of a square table, with cushions on all 4 sides (but no rails), and butt two of them up against each other. For a 50" playing surface and 2" cushions, that produces a length of 108", a "9-footer." Again, that's a "nominal" length, not any real dimension of the table.
The standard 7-footers (38" x 76" playing surface) and 8-footers (44" x 88" playing surface) can be thought of the same way:
(38 + 4) x 2 = 84
(44 + 4) x 2 = 96
The "odd" sizes of playing surfaces, obviously, do not produce 7-. 8-, or 9-foot "nominal" sizes under that formula.