[crote]

In almost every instance of stuff like that it is caused by the manufacturer not reading the specifications and implementing it improperly. Usually the cause is them trying to save literally $0.001 by leaving out a component they think is "optional".

The specification is very clear with regards to chargers: a charger with more watts is always better than one with fewer watts. For any pair of chargers, if charger A provides X watt and charger B provides Y watt, if Y > X every device which can be charged by A can also be charged by B. This means a laptop charger will always also charge your smartphone - although the opposite might not be true due to the laptop having a minimum power requirement.