Picky Perkins--
The NY Times of Feb. 5 ran an article summarizing the paper you cite, under the title
NASA Was Told in 1990 About Vulnerable Protective Tiles Here's the URL:
http://www.nytimes.com/2003/02/05/na...t&position=top
The article, which is well worth reading, focuses on the wheel wells, pointing out why they are potential trouble areas, but it also explains by implication why the diagram of danger areas is asymmetrical. (By the way--the Times article has a similar but simplified diagram of the shuttle, but the paper it cites, by the authors you name, is dated 1990. A minor mystery, probably not worth puzzling over.)
The Times authors say,
"Tile damage in the wheel areas was relatively high, as it was along the whole right side of the underbelly. That, they found, was because foam insulation had flown off two long pipes that run alongside the external fuel tank."
In addition,
"To the data on tile damage, the scientists added temperature readings of where the shuttle tiles became hottest and were most likely to burn through. Dr. Paté-Cornell said the wheel areas got unusually hot during re-entry because their doors created a 'discontinuity' in the tiles that would disrupt the smooth flow of air over the shuttle's belly."
The authors of the cited paper then correlated the tile-damaged and high-heat areas with vulnerable parts of the shuttle: places where hydraulic lines ran underneath, as well as sites of hydrogen and oxygen tanks.