Distribution Graph shows 1 cluster when K-Means generates more than 1 cluster.
Generated a K-Means nugget and it provides expected data when reviewing (editing) the model. When attaching a Distribution Graph output to this nugget and running, only one cluster is appearing within the 'Table' tab.
Not all clusters appear within Distribution Graph output.
Resolving the problem
Applying Modeler 15 FP1 resolved the issue.