In the API is noted that only widgets and validators are merged:
sfForm::mergeForm ($form)
$form The sfForm instance to merge with current form
Merges current form widget and validator schemas with the ones from the sfForm object passed as parameter
throws LogicException If one of the form has already been bound
In the code it appears that defaults are also merged, which is not stated in the documentation.
What I expected was that mergeForm would also merge the labels.
Solution is to add this line to sfForm::mergeForm :
$this->getWidgetSchema()->setLabels(array_merge($this->getWidgetSchema()->getLabels(),$form->getWidgetSchema()->getLabels()));
