Surefire random number of tests

[Igor Petruk]

Here is my SureFire config

                <plugin>
                    <groupId>org.apache.maven.plugins</groupId>
                    <artifactId>maven-surefire-plugin</artifactId>
                    <version>2.8.1</version>
                        <dependencies>
                          <dependency>
                            <groupId>org.apache.maven.surefire</groupId>
                            <artifactId>surefire-junit47</artifactId>
                            <version>2.8.1</version>
                          </dependency>
                        </dependencies>

                    <configuration>
                        <parallel>none</parallel>
                        <forkMode>once</forkMode>
                        <threadCount>4</threadCount>
                        <excludes>
                            <exclude>**/manual/**</exclude>
                        </excludes>
                        <argLine>-Xmx1024m -XX:MaxPermSize=128m</argLine>
                    </configuration>
                </plugin>

When I run my test set I get different number of tests executed. When the 
"parallel"="none" I get about the half, but different each time. If I enable 
parallel test execution then the number of tests is smaller.

What could be an issue? How does forkMode relates to this issue?

Spring 3.0.5, JUnit 4.8.2