The NServiceBus Saga Persister uses different conventions for storing sagas than the Raven default conventions. The Raven default convention is to take the class name and pluralize it. Apparently NServiceBus does not do so. So you have a mismatch by accessing the data in fundamentally different ways.
You may want to reuse the NServiceBus Saga Persister class. The Saga persistence code can be found here:
https://github.com/Particular/NServiceBus/tree/4.2.0/src/NServiceBus.Core/Persistence/Raven