It seems recently that bosses get tweaked before anyone can be sure of their status as mathematically unkillable, especially with the way that gear works now.
Unkillable bosses would require encounter breaking bugs or be so overtuned that it's obvious that even with BiS-pre-that-boss gear it would never fall.
In both of these cases the hotfixes normally arrive pretty quickly.
Tuning has been pretty decent for the majority of fights recently, and it makes logical sense to overtune a fight.
If you undertune a boss and loads of people kill it before you go in and change it then it's pretty harsh on the people who didn't kill it before the change. They're then left with less gear than the other guys to kill a harder boss next week.