Besides licensed content, the COUNTER usage statistics will show usage of all content that is freely available on SpringerLink/Springer for R&D. This will include usage from: Open Access Journals, open choice articles in hybrid journals and usage from other journals that are freely available (for example newly launched journals).
Another reason could be that some content has temporarily been made freely available due to a marketing campaign etc.
Besides the above, another reason would be multiple recognition of users: For example: there may be instances that a user logs in with their username & password, but is from a different institution that also has a Springer account and access rights. In these cases, the user will have BOTH the access rights from the account via IP recognition at the guest (your) institution, AND the access rights they have due to the recognition via user name and password from their parent institution. In such cases, the usage generated will count for both accounts.