I've come to understand finally that Miles & More offers greater availability to their own members than they do to the rest of *A. And Expertflyer is showing M&M availability, apparently.
There often ARE IT syhching issues between US and their partners. But the inside availability vs outside availability is an issue with some carriers. SQ also offers their own members greater inventory than their partners, for instance (and in the case of MUC-SIN, sometimes less availability!).