When I first create a new job and add my Builder to the build steps, the f:repeatable appears empty (see no-elements.png). I save that configuration and Hudson produces this config.xml:

<?xml version='1.0' encoding='UTF-8'?>
<project>
<actions/>
<description></description>
<keepDependencies>false</keepDependencies>
<properties/>
<scm class="hudson.scm.NullSCM"/>
<canRoam>true</canRoam>
<disabled>false</disabled>
<blockBuildWhenUpstreamBuilding>false</blockBuildWhenUpstreamBuilding>
<triggers class="vector"/>
<concurrentBuild>false</concurrentBuild>
<builders>
<com.example.HelloWorldBuilder/>
</builders>
<publishers/>
<buildWrappers/>
</project>

When I reload the job/test/configure page the f:repeatable component all of the sudden contains one element (see one-element.png). This should not happen unless the config.xml looks something like:

<?xml version='1.0' encoding='UTF-8'?>
<project>
<actions/>
<description></description>
<keepDependencies>false</keepDependencies>
<properties/>
<scm class="hudson.scm.NullSCM"/>
<canRoam>true</canRoam>
<disabled>false</disabled>
<blockBuildWhenUpstreamBuilding>false</blockBuildWhenUpstreamBuilding>
<triggers class="vector"/>
<concurrentBuild>false</concurrentBuild>
<builders>
<com.example.HelloWorldBuilder>
<configurables>
<com.example.HelloWorldBuilder_-Configurable>
<selectedOption></selectedOption>
</com.example.HelloWorldBuilder_-Configurable>
</configurables>
</com.example.HelloWorldBuilder>
</builders>
<publishers/>
<buildWrappers/>
</project>

Sample plugin code can be found at: http://github.com/plouj/testHudsonSelect/tree/350ad201c209ba393d6a3d417b34a9ceac466ba1

Steps to reproduce:
1 - build and plugin
cd testRecorder && mvnDebug hpi:run
2 - create a new free-style project named "test"
3 - add a "Test selection" build step
on the initial job configuration page click "Add build step"
select "Test selection"
4 - notice that the "Configurables" section correctly contains nothing other than an "Add" button (see no-elements.png)
5 - just save the job configuration
6 - verify that the job config.xml file just says:
...
<builders>
<com.example.HelloWorldBuilder/>
</builders>
...
without any sub-elements
7 - reload the job configuration page
browse to job/test/configure
8 - notice that now the "Configurables" section incorrectly contains a dropdown menu labeled "Select an option:" besides the "Add" button (see one-element.png). This represents an element which has not been read from the disk (config.xml).
9 - for a sanity check save the job again
10 - notice that config.xml now shows one "configurable" element as expected:
...
<builders>
<com.example.HelloWorldBuilder>
<configurables>
<com.example.HelloWorldBuilder_-Configurable>
<selectedOption></selectedOption>
</com.example.HelloWorldBuilder_-Configurable>
</configurables>
</com.example.HelloWorldBuilder>
</builders>
...