The health the ur dragon has is constantly adjusted according to what the potentially hundreds of players are doing and how they're performing. The weakspots destroyed is, as I've said, based on it's % of health. As it gets closer to death, the more inaccessible spots are the ones left going, with finally the ur dragon spawning with no health but all weak spots active for the player to strike the deathblow.
The damage the players do is pooled, and calculated serverside.
Keep in mind that the ur dragon also heals when people lose. This doesn't necessarily mean noobs are fighting- this means more likely that players are joining to see the health, seeing it's not dead, and fsing out. Ferrystoning (fsing) counts as a loss against ur dragon. Thus, this is the cause of it's health fluctuations.