Click on either the actual '50' or the '100' and you will see that your contacts will be checked - being either the first 50 or first 100 that are selected.
The 50 and 100 have been underlined as shown above in red.
If you click on 50 you will select the first 50 contacts shown in List view - as shown below:
If you click on the 100 instead it will select the first 100 contacts.
If instead you click on the checkbox itself at the top you will see that ALL contacts in List view are selected as shown below: