Because no two cars or engines are identical!
The largest manufacturers in the world have natural variance during the manufacturing process – so it can only be realistically expected that variances would exist in the real world over time.
No two setup can be compared due to the infinite number of variables present at every opportunity – information and results can be used as an indicative guide, but in the world of high performance modified cars, the variations are what create the uniqueness to the scene anyway – embrace it – not get upset because a magazine or internet forum somewhere suggested a different result.