What is "No Tenancy" in Cost Explorer?

I was getting two very different sets of numbers in Cost Explorer between the Cost Explorer details linked from the EC2 Reserved Instances Recommendations, and doing my own filtering based on known instance types in tags in another window (costs and hours in my window were about double).

The filter that made them match up turned out to be Tenancy, where the RI page was specifically specifying Shared Tenancy, but I had no filter on, which was including both Shared and No Tenancy.

What does No Tenancy mean? I can find info on Dedicated, Shared and Default, but not "No Tenancy".

Note, there are no dedicated hosts being used.