Why is cross-country time defined differently for different certificates?

Each authority defines XC by purpose: for currency or aeronautical experience, any landing at a different aerodrome may count; for certificate eligibility (e.g. FAA PPL/IR/ATP, EASA modular CPL) a minimum straight-line distance from departure (typically 50 nm for FAA, 100 nm for some EASA cases) is required. Always log the actual distance and route so you can claim time under the appropriate rule.