The leaderboard system for Heathstone's Arena mode is to be adjusted for October and November. The news was confirmed by principal designer Mike Donais on Reddit yesterday evening.
Typically the leaderboards are determined by whoever achieves the most wins per run (on average) across 30 consecutive Arena runs. For the months of October and November though, that's going to be reduced to 20 consecutive runs.
The reason for this change relates to the tweaked version of Arena that's currently in place to celebrate the Halloween season. Here - and for a period of two weeks only - players are able to select an alternative hero power after choosing their class and commencing their deck draft.
While it's a lot of fun as a temporary seasonal tweak, it also unbalances the Arena format to such a degree that it would be unfair to mix the results in with the regular Arena stats.
Instead, the so-called dual-class arena mode will have its own leaderboard based on the best performances across 15 consecutive runs, and the usual monthly leaderboards will have lower run requirements in roder to reflect the shorter duration of the usual competitive seasons.