... so as you say extra DECK or just DECK are two different thing
the why
no one can activate compulsive evacuation device on a synchro monsters if gb hunter is on the field .... ( GB hunter stops from sending cards to the deck)
and in the same situation mirror of oath cant be activated on gyzarus summon .....
on one card deck and extra deck is one and the same thing but on other card its needed exactly main deck ....