Do Maven project properties have write-once semantics - maven

Question

I observed a Maven behavior today that left me puzzled. Either the project properties have some sort of write-once semantics or... I failed to find any reference documentation that would explain this behavior.
Can I dynamically set Maven properties in the validate phase to have them evaluated as plugin properties in later phases?
Goal
I want to skip the execution of a plugin in a profile based on the existence of some other property.
Plan
Run the Groovy plugin in the validate phase.
Evaluate the existence of that other property.
Set the skip project property.
Use the skip property for the configuration of plugins that run in later phases.
POM excerpt
<profile>
<id>my-id</id>
<activation>
<file>
<exists>${basedir}/somefile</exists>
</file>
</activation>
<properties>
<skipIt>false</skipIt> <!-- intention: use this as default value -->
</properties>
<build>
<plugins>
<plugin>
<groupId>org.codehaus.gmavenplus</groupId>
<artifactId>gmavenplus-plugin</artifactId>
<executions>
<execution>
<phase>validate</phase>
<goals>
<goal>execute</goal>
</goals>
<configuration>
<scripts>
<script>
<![CDATA[
def someOtherProp = project.properties['some-other-property']
def skipIt = someOtherProp ? 'false' : 'true'
project.properties['skipIt'] = skipIt
println skipIt
println project.properties['skipIt']
]]>
</script>
</scripts>
</configuration>
</execution>
</executions>
</plugin>
<plugin>
<artifactId>maven-dependency-plugin</artifactId>
<configuration>
<skip>${skipIt}</skip>
</configuration>
<executions>
<execution>
<id>my-unpack</id>
<phase>process-resources</phase>
<goals>
<goal>unpack</goal>
</goals>
Behavior
If some-other-property does not exist (i.e. is not set) this prints true twice. That means the Groovy evaluation is correct and skipIt was correctly set. Yet, the dependency-plugin still runs the my-unpack execution. Why?
However, if I remove <skipIt>false</skipIt> on line 9 everything works as I expect. Why?