It appears that individual emails within a conversation can be inconsistently labelled - I think this arises when using GMail on Android where individual messages can be labelled.
This problem arose when I searched for "mykeyword -label:mylabel" and conversations showed up which WERE ALREADY labelled with mylabel. SOME of the messages WITHIN the conversation were not labelled with mylabel, so the entire conversation showed up.
Is there a way to review all the label variation within a conversation and conform them? Right now, I have to unlabel and relabel the conversation.