You see, what i don't get is why Nexon gives themselves 1:30 mins for maintenance when it turns out to take almost 8 hours.

Why not just give yourself extra time and end the maintenance early? 

They probably didn't know it would take the 8 hours.

It makes sense that they originally did intend to do it in 1h30, but in the end, problems further come up that need to be addressed.