What is difference between network service and network interface
networksetup is the CLI interface for network settings panel in System Preferences.  From the man page:


  The networksetup command is used to configure network settings
  typically configured in the System Preferences application.


ifconfig is the utility to set up a network interface.  From the man page


  The ifconfig utility is used to assign an address to a network
  interface and/or configure network interface parameters.


What you are seeing is two different methods with different nomenclature for the same thing.

networksetup is specific to macOS where ifconfig is more generic and part of the BSD core

As to your question...


  Can a single network interface have multiple network services?


Yes.  For instance, you can have an alias for that interface.  Using actual System Prefs (because it's visually easier), I created a new service called "Ask Different Demo" to my existing interface "Ethernet"