Storyline Heat

Can someone explain storyline heat to me, please, as it I don't think it's covered in the TEW IX handbook. I currently have a Shawn Michaels v Triple H feud going. It started two weeks ago and has had two major advancements. The first was rated 80. The second 74. There have been no other segments advancing this storyline. However, my heat is 65. I don't understand why the heat is so low when the segments have received good ratings.