Different types of USB cable
The older USB cables from iPod days have deeper and thicker plastic enclosures on the 30 pin end as well as physical buttons on the sides to help disengage the metal "ratchets".

The connectors that came with the iPhones had much weaker springs for the side "ratchets" so you could just pull to detach the connector.



Here is a Bluetooth iPhone charger (that is slightly larger and thicker than the iPod model) with a current iPhone cable for comparison.



The cables are all functionally equivalent once seated so you don't need to worry about current or power since even the oldest USB cable can carry all the signals needed for the latest iOS products and chargers.