Why do some users have an association bonus even if they are not in any other Stack Exchange sites (as shown in their profile)?
They may have deleted an older account which qualified them for the association bonus. Deleting such an account does not remove the association bonus on your remaining accounts, and it's even the case that your network account is marked as qualifying for the association bonus. That means when this user creates a new account on another Stack Exchange site, they'll get the association bonus too.
They also might have hidden the account you're looking at from their network profile, or have hidden all others. Those accounts will be shown as having no linked accounts too.
You can choose to show/hide some of your communities on your profile. To do so, visit your profile, and then under your communities list, click on "Edit list":
Alternatively, you may visit this link.
Those two users are most likely registered in other communities but they chose not to show them.