The slant is to allow big payloads to be taken out with adequate clearance:
Meanwhile, the ISS design had to accommodate the shuttle. It needed to provide a zigzag tunnel mechanism (the Pressurized Mating Adapter) to optimize the clearance to remove payloads from the bay after the shuttle had docked.
— Hale, Wayne, and Helen Woods Lane, eds. Wings in Orbit: Scientific and Engineering Legacies of the Space Shuttle 1971–2010. Government Printing Office, 2010. p. 138.
(Links: homepage - text - PDF - ntrs.nasa.gov)
Note: that book isn't just any book; see this part of its forward:
We are grateful to all the institutions and people that worked on the book. (See appendix for complete list.) Each NASA field center and Headquarters contributed to it, along with many NASA retirees and industry/academic experts. There are a few who made exceptional contributions (...)
— Space Shuttle Flight 111 (STS-112) Post Flight Presentation (YouTube)
A finding worth mentioning: that PMA's basic design is from Space Station Freedom (SSF) Level 2 Program Office, a 1993 derivative of which was considered for Soyuzes docking to SSF as Assured Crew Return Vehicles (ACRV) after reaching the Permanently Crewed Capability (PCC) construction stage.
— Cruz, Jonathan, Marston J. Gould, and Eric Dahlstrom. Soyuz/ACRV accommodation study. National Aeronautics and Space Administration, Langley Research Center, 1993. p. 147. https://core.ac.uk/display/42789859