Get report vs get reports

I'm performing this request:

GET https://api.powerbi.com/v1.0/myorg/reports

or 

GET https://api.powerbi.com/v1.0/myorg/groups/{groupId}/reports

And I use one of this id's to perform this request: 

GET https://api.powerbi.com/v1.0/myorg/reports/{reportKey}

 On the first, I get a response as expected, but when I perform the second request I get a PowerBINotAuthorizedException.

Can anybody explain why a list is allowed, but a single entry isn't?