Freelancer has all 5 star reviews, but low JSS?

[#1]

I am seeing freelancers with apparently good reviews, at least from what is visible to the client.

Yet a JSS of <70%. I am confused about what this means. Can anyone help me understand it?

[#2]

It probably means they had to refund one or more contracts to the client fully.  This hurts JSS and the feedback does not appear on the profile.

[#3]

The stars don't matter much for JSS. It's based more on the private feedback which is structured more like a net promoter score. It's possible one or more of the 5-star reviews had poor private feedback. 

The other possibility is what Jason mentioned, that there are closed or dormant contracts with no money paid on them.