I created a new user (ID=a0001) in my local AEM and both functions give me the same value (which is a0001).
The Adobe reference document is not very clear on the distinction. Thanks
EDIT: I had a look in CRX/DE for the properties of a0002 user and rep:principleName and rep:authorizableId has the same value.
String principalName = userProperties.getPrincipalName();
String authorizableID = userProperties.getAuthorizableID();