The order of cin operations must match the order users type values. If you write cin >> a >> b; but the user types 20 10, then a becomes 20 and b becomes 10. If you swap the variable order in code to cin >> b >> a; with the same input 20 10, now b becomes 20 and a becomes 10.
Variables fill in the order they appear in the chain. This matters with mixed types: int age; double salary; cin >> age >> salary; expects age first. If the user types 50000.50 25, your age gets 50000 (truncated), producing incorrect results.