Why the number of tests run in parallel is more than in single thread

[Mao, James(Maode)]

Hi,

I am using Surefire plugin 2.17 with the following configuration

<configuration>
         <parallel>classes</parallel>
         <forkMode>perthread</forkMode>
         <threadCount>4</threadCount>
         <reuseForks>true</reuseForks>
         <perCoreThreadCount>true</perCoreThreadCount>
</configuration>

Previously the tests run in 2 min, now it finished in 30 seconds,

However, with single thread, from the summary log, Tests run: 160, Failures: 0, 
Errors: 0, Skipped: 4, the number of tests run stable at 160.

But with multiple threads, the summary log shows Tests run: 1530, Failures: 0, 
Errors: 0, Skipped: 14, the number of tests run also not stable, sometimes more 
than 1530, sometimes less than 1400.

Is this the summary report issue? Or the it does run multiple times for a 
specific tests?

Thanks,
James