I just noticed that the D&D monsters are made according to a strict formula. They are assigned a level, a role, statistics, and powers, then hit points, defenses, and attack and damage bonuses are calculated based on level, role, and statistics. The level of the monster is an input, not a calculated value; it is not affected by how good the monster’s statistics and powers are. This would explain why the effectiveness of many monsters seems to vary greatly from the level of the monster.
The needlefang drake swarm, for instance, is described as level 2 because it uses the basic statistics of a level 2 monster. The unbelievable offensive power provided by its special abilities is not taken into account. The zombie, on the other hand, is also considered level 2, but its "special power" of having incredibly low stats makes it very weak.
I'm estimating that the needlefang drake swarm is effective level 6 and the zombie is effective level 1 (and on the low side of that).