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I knew I'd seen this. And that if I kept on going through the Whitbread Gravity Book I'd come across it. The size of the cells used by Whitbread for their colour measurements.

Take a look:

The SB Pale Ale has 1" cell (that's one inch) written above the colour value of 32. Brown Stout has 1/16" (one-sixteenth of an inch) above it's value of 1 + 17.

That explains why the numbers for Stouts always look way too low: because the measurements were made using a much smaller cell than for paler beers.

Glad that's been explained.