Simple answer:
For N1 rated engines (GE), in Normal mode the FADEC calculates the max N1 rating for the ambient conditions (temp, Mach/airspeed, altitude). Full forward thrust lever position will give you that max rated thrust (plus a few tens of a percent "headroom"). You can't have a significant overboost in Normal mode.
In Alternate Mode, the FADEC doesn't calculate the max rating (typically because reliable Mach/airspeed is unavailable - the FADEC requires two sources of Mach/airspeed that agree (within a small tolerance) - on the 777 that means both ADIRU and SAARU as the GE FADEC doesn't have an independent source of total pressure). Instead, the FADEC calculates a max N1 based on the corner point temperature for that altitude - that becomes the full forward thrust lever position, and that's used to schedule the N1 for lower thrust lever positions. Depending on the actual temperature, you can command a significant overboost relative to the actual max N1 for those conditions.