What's the difference between Zone and AFN in output variable?

I'm deriving several outputs from my model to evaluate indoor air quality.

What is the difference below? There is only a slight difference in the result value.

'Zone Air CO2 Concentration' vs. 'AFN Node CO2 Concentration'
'Zone Air Generic Air Contaminant Concentration' vs. 'AFN Node Generic Air Contaminant Concentration'
'Zone Air Humidity Ratio' vs. 'AFN Node Humidity Ratio'

There is one more question. When setting the output variable to 'AFN node', why do I need to input 'zone name' instead of 'surface name' in key value? Is it an error in Energyplus?

The 'Zone Air ###' outputs are the average value for a zone and the 'AFN Node ###' outputs are generated for each node in the airflow network that you define with an AirflowNetwork:Distribution:Node object. Taking the CO2 Concentration example that you referenced, here is the documentation for 'Zone Air' and here is the documentation for 'AFN Node'.

The key value for the 'AFN Node' outputs should be the names of the AirflowNetwork:Distribution:Node objects in your model. One might match the name of each zone for the zone air node, but you will also have nodes after coils, fans, and other HVAC components in the airflow network. I don't think there are AFN nodes associated with surface names. If you set the key value to an asterisk "*" then EnergyPlus will generate all possible outputs for the variable you request.