(a) Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) said that the Prophet (peace and blessings be upon him) and his Companions performed only one tawaf between Safa and Marwah, their first tawaf.
(b) This is because the Prophet (peace and blessings be upon him) was performing Hajj alone (mufrid), and some of the Companions were combining Hajj and Umrah (qarin), so they performed one sa'y. Sayyidah Aishah (may Allah be pleased with her) had added Hajj on top of her Umrah, and she did not perform tawaf or the sa'y of Safa and Marwah before Arafah. After that she performed tawaf and the sa'y of Safa and Marwah, and the Messenger of Allah (peace and blessings be upon him) said to her the following:
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9418