Actually it's (should be) basic ATPL knowledge.
If you set the thrust levers to the same position at take offs with different temperature (all other factors remaining equal), you will get different thrust outputs. Cold = more thrust, warm = less thrust.
To get e.g. 27000 lbf thust from each engine, the thrust lever has to be set differently depending on temperature.
The temperature at which the engine is "flat rated" is the highest temperature where it is able to produce the maximum thrust. Normally as said ISA + 15 degs (30 deg C ambient temperature).
Below 30 degs the engine is able to produce more than max (e.g. 27k). At 0 degs C it may be able to produce 30k (as an example only) without exceeding any internal limits, like EGT, fan speed and whatever other limit that an engineer may tell you about.
However, the max allowable thrust also depends on other things, like e.g. engine mountings (probably a lot of other things too, which again an engineer may know lots about).
So at 0 degs C, instead of exceeding max thrust, the engines has to run at a lower speed. Hence, a lesser thrust lever setting.
Above 30 degs C, the engines will no longer be able to produce max thrust. As in doing so, an internal limit will be exceeded. If e.g. EGT is the limiting factor, thrust has to be reduced to keep it within limit, hence the output will be less than max.
The EEC is, through its sensors, able to calculate what the thrust lever angle should be according to the temperature in order not to exceed any engine limits or max thrust according to above. This calculation output is presented to the pilot as the "Reference N1 bugs" on the primary engine indications display.