I have a feeling that its something to do with pressure.
If the engine is pressure rated (ie certified to a max pressure) then the max N1 setting is different for different pressure altitudes. At SLP it could be 100% but at 10,000' it could be 102%, 20,000 104% etc.
The FMC can calculate all this and so sets its limit (red line on the gauges) if im not mistaken firewalling the thrust levers will give you to the red line at any altitude/phase of flight.
Im not sure you can overstress the engines with the EEC's in normal mode as there is a protection system on the -800
Please do correct me as I want to learn too!
edited to add the EEC's comment