Why isn´t TOGA thrust always 100% N1 or N2?
TOGA thrust is not always 100% N1 (or N2), because TOGA thrust is actually not about delivering the absolute maximum thrust that can be provided. Rather, it's about delivering a guaranteed large amount of thrust, to ensure the performance of the aircraft is what the pilot expects, and what the flight manual states will occur. The level of thrust needs to be predictable. So it's got to be the same with a new aircraft, as one 5 years old (for example), and the engines are about to be overhauled.

To achieve this, engines are "flat rated". See What is a Flat Rated Engine?



To repeat that answer a bit, a flat rated engine, produces a constant, or the same amount of thrust, up to a certain ambient temperature. This is achieved by having a margin (gap) between the maximum thrust the engine provides, and what it can really produce. This also results in a temperature margin between the engines actual turbine temperature, and the red line temperature. This means as the engine degrades, it can increase its operating temperature without hitting the red line (until it has degraded a lot), and so still provide the same level of thrust.

When a gas turbine's engine control is designed to produce the same thrust at different ambient temperatures, it means N1 speed will be higher on a hot day, than a cold day. See how in the figure, for a constant thrust, N1 is increasing as ambient temperature goes up? N1 has to go up as ambient temperature increases, because hotter air at the intake is less dense than colder air, so the engine needs to run at a faster RPM to make up for the lower density.