The reason for this is that newer versions of Report Designer use newer features that older versions of the Engine don't recognize. 

The best practice is to keep your version of Report Designer below or equal to the version of the Engine. The problem with using a new version of Report Designer, and an older version of the Engine is that you could possibly create features that the Engine is not aware of in your template.  It will run fine in Report Designer, but when the Engine encounters a new or altered feature, it will not be able to process it or it will process it differently than expected.