International Hose standards specify a set of materials and tolerances, such as internal and external dimensions and reinforcement types and patterns. The reality is that while these standards are adhered to by all manufacturers, the tolerances themselves are so broad that if the entire allowable tolerance was used in manufacturing, users would encounter a high failure rate due to hose and fitting tolerance mismatching. High quality hose manufacturers have to adopt their own tolerance limits which are often at least half of the allowed range.
This is where “Mixing and Matching” becomes an issue: Component manufacturer ‘A’ could produce parts on the lower limits of the tolerance, and manufacturer ‘B’ is on the upper end of the tolerance. If a fitting from ‘B’ was put on a hose from ‘A’ at the specified crimp diameter of ‘B’, there would be little chance of adequate fitting retention, which would most likely result in failure. Similarly if a fitting from ‘A’ was assembled to a hose from ‘B’ the likelihood is that the inner tube of the hose would be over-compressed or the hose reinforcement could be cut, again resulting in premature failure.