I am not 100% sure of this, but I believe what you are experiencing is because of the way xDB handles identified and unidentified contacts.
From Identifying Contacts, we learn:
Every time a contact uses a device to interact with your website xDB
creates a new contact record. New contacts enter the system as
unidentified contacts because the only information collected at this
point relates to the browser or device used. This is not enough to
identify an individual across multiple devices.
Contacts are connected to the devices they have been using but remain
unidentified in the system until they provide a valid identifier.
Clearing cookies in the browser breaks the connection between the
contact and the device.
Depending on your requirements, there are different identification
strategies that you can implement to identify contacts across their
devices. For example:
- Introduce a login form.
- Connect to contacts using their given email address or phone number and SMS authentication. These all require you to integrate with the Sitecore platform calling the Identify() API call when applicable.
- Email Experience Manager (EXM) and the Web Forms
for Marketers (WFFM) modules integrate with Sitecore to identify
contacts when they open email links or complete a web form.
contact information, as well as an identified contact's history, can
be accessed across multiple devices. When a contact identifies
themselves using a particular device or browser, Sitecore stores the
connection details and, by default, retains this association for the
subsequent sessions that follow.
So to point this to what you are experiencing, I believe you may have 120 contacts in total, 64 of which are identified - and only the identified ones make sense to pull up in a report.