The transverse contrast uses toy visible fractions, not fitted decay constants. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Toy transverse fractions are visible fractions
The brighter row keeps 9 of 12, or 3/4.
9/12=43
The dim row keeps one half
The contrast row keeps 6 of 12, or 1/2. No real transverse decay constant is being fitted.