Custom Plugin executed several times depending on @phase - why ?

[Houze Christian-Luc \(DCTI\)]

I have created a very simple custom plugin (hello world)
Depending on my mojo specified "@phase" it is executed once or more
(I run maven with "mvn install" on a project with a pom that has "jar"
packaging)
Exemple:

@phase process-classes one execution for "hello world"

@phase validate  three executions for "hello world"

Why ? 

Thanks for any hint

                Christian-Luc