Jammedstab, the explanation is simple. Don't confuse overboost with overspeed.
It has nothing to do with alternate mode (other than the fact that you need to be in alternate mode to significantly overboost the engine).
I'm assuming you're talking PW4000 (GE and Rolls have different overspeed protection). If everything is working normally, you should NEVER exceed N1 or N2 redline - the EEC is programed to protect those limits. So if you do exceed redline by more than a fraction of a percent, it means something's broke and the EEC can't control the engine. In order to protect structural limits, if the EEC senses an N2 overspeed greater than a threshold (I don't recall the specifics but 5% sounds about right) it trips the overspeed protection system which forces the fuel metering valve to the min flow stop (which is sub idle except at very high altitudes).
Last edited by tdracer; 5th Nov 2014 at 18:55.
Reason: Edited after re-reading the original post