NUnit tests marked [Ignore] sometimes included in the Tests Passed number

We are seeing interesting behavior with the [Ignore] attribute in our NUnit tests when run through TeamCity.  We have four build agents all running on one machine; for two of those, we see the [Ignore]d test counted under "Not Run" tests, but is included among the number that passed (in this case, 49).  For the other two agents, there are 0 tests Not Run, and we see 48 tests passed.  Running the NUnitLauncher on the command line for each of the build agents (as the NUnitLauncher is agent-specific) against the test DLL built by the various agents, the behavior appears to be dependent on the test DLL itself rather than the NUnitLauncher used.  What might cause the [Ignore] attribute to be treated differently in this way depending on which agent builds the test DLL?