LogFAQs > #975897210

Janine is a gym leader who got the position because her dad was inexplicably promoted to Elite 4. Guessing Agatha retired and they were desperate for the first poison type user they could find.
Janine uses Ariados. Not one, but two of them. On purpose.
Koga had an Ariados too now that I think of it.

Maybe in Pokemon land, you're sworn to uphold your family's dignity by insisting the Pokemon your ancestors used was and still is the best.
Would explain the rival getting as far away from using ground types as possible.

Oh actually in the English version, maybe it goes deeper. In Japanese I know they're named stuff like Kasumi and Takeshi, regular names. In English, rock trainers just so happen to be named Brock. A girl named Roxanne too. A steel trainer named his son Roark, probably with the type advantage on his mind.

"I really enjoyed the battle with you!" - Brock, who was hit with 5 consecutive 4x damage Giga Drains and did not get a turn.
Maybe he hates his Pokemon. Maybe he's insane. I know I just got on Koga and Janine's case about using Ariados, but Brock used Graveler. Something's up.