When I removed mine several years ago the thread was damaged and I originally thought someone had put a metric bolt in there as the pitch was wrong but then worked out it had been stretched.
The specification they give is for a grade5-5/8unf bolt as indicated by the chart in the manual, but the correct bolt is only a grade5-9/16unf so the lower torque specification is correct.
I replaced my damaged bolt with a grade8-9/16 and used a higher torque suitable for that bolt.
http://www.mpi-online.com/technical/torquechart.htmHere is a torque chart that might be of some interest.