At the beginning of the Shemona Esrei for both Shacharit and Maariv, one recites the words "אדני שפתי תפתח ופי יגיד תהלתיך". At the beginning of the Shemona Esrei for both Mussaf and Mincha, however, one precedes those words with, "כי שם ה' אקרא הבו גודל לאלקינו".
Why is it specifically by Mussaf and Mincha that those words are added?
