Not quite sure if this applies to VS2010. In VS2008, running tests in the current context sometimes runs all the tests in a solution. It seems that the context is determined by which element was last activated (usually with the mouse). If the last window is selected, these are code windows containing the unit test source code, then all tests in this class / file are executed. Same thing with selecting a file (unit test -) in the solution explorer. If, however, the last time other windows were selected (for example, breakpoints), then the context is the whole solution.
source share