Hi, thanks for posting!
This is behaviour by design. The _data property is queried outside the scope of the test, as NUnit uses this property in order to analyse and identify the test (which is a step that occurs prior to any execution). If this is messing with your coverage metrics and making your code look bad, I recommend suppressing the coverage for this property with an '//ncrunch: no coverage' comment.
Cheers,
Remco