So, I just happen to have a copy of the 208B EX POM infront of me, I was also intrigued by this question and did some research but couldn't find any particular reason for this limit after a bit of googling around but did notice the following Note under Maximum Engine Torque for Climb it says:
1. Torque on this chart shall be achieved without exceeding 825c ITT or 103.7 percent Ng
Where as for the "Cruise Maximum Torque" it states:
1. The Highest Torque shown for each temperature and RPM corresponds to maximum allowable cruise power. Do not exceed this torque, 805c ITT, or 103.7% Ng, whichever occurs first
As the Maximum Engine Torque for Climb doesn't mention the line "Do not exceed this torque" unlike the Cruise Maximum Torque section and does mention the ITT and Percent Ng not to exceed I'd say that OCTA and others are correct and it's merely a simple way to ensure you don't exceed your ITT or Ng limits with less fiddling around than simply setting whatever gives you just under the limits.