At the moment NCrunch's impact detection is operating at a very basic level. When you make a change to a source file, it will examine the file to see which tests cover it, then consider all of these tests as 'impacted'.

This behaviour is because NCrunch doesn't have an understanding of the programming language itself (C#/VB), since it doesn't physically parse it. The tool is instead built on top of other abstractions above the language level (such as MSBuild), so the 'flat-file' impact detection is just a best effort to try and prioritise tests more intelligently without tying the engine down in expensive deep impact analysis.

There are new opportunities appearing in this area with the production release of Roslyn later this year. I hold high hopes that these opportunities will enable far superior impact detection features within NCrunch.