Just typed up a long reply and ended up clicking mouse and lost everything, so here goes another shot.
Your question will take a few posts to answer, so I will get it started.
If you refer to CEC 4-004, there are a number of rules that refer to derating and correction values applied to conductors. Generally, when you have excess heat, you will have to apply correction factors to the current carrying capacity of the conductors.
For example, excess heat could come from higher than normal ambient temperatures that may be found in some industrial environments. Also, excess heat can come from other conductors such as in a raceway or cable tray. In those cases you would refer to the tables 5A,B,C or D for the appropriate value to derate the conductors by.
Keep in mind, if you are encountering a retrofit or addition to existing, you may have to apply correction factors to the existing work such as pulling another cable in a cable tray. Then you would have to review the ampacity of the existing conductors if you wanted to use the same tray.
In the example you mentioned, there may have been no need at all for derating, or the original install may have been incorrect.
