נֻסח 'ספרד' says "אין כאלקינו" every day. On weekdays, it includes several paragraphs describing the makeup of the קטרת, followed by several פסוקים. On שבת, the description of the קטרת is cut short and the balance, along with the פסוקים, is replaced by a משנה from מסכת תמיד. Also, the very paragraph of "אין כאלקינו" is slightly different: its ending changes for שבת. Why these differences between weekdays and שבת?