D&D 3E/3.5 [3.5] Hellwasp Swarm question

[The Spectrum Rider]

Under Special Qualities, the Hellwasp Swarm has both "damage reduction 10/magic" and "immune to weapon damage."

Does this mean anything? If it's immune to weapon damage, what difference does the damage reduction make?

If no one can clear this up, I'll send it in to the erratta.

The Spectrum Rider

 

[AuraSeer]

There's no contradiction. The swarm qualities are in addition to those of the individual creatures.

If you captured a single hellwasp out of the bunch, it would have DR 10/magic. When they're running around in a swarm, the swarm's special immunity makes the DR irrelevant.