I read this post that only merit source have expiration for smerits. But yet only a proposal I guess. Confuse too with this.
You're right, only merit sources have their smerit count replenished monthly and unused smerits from the previous month would be decayed. At this point it doesn't apply to regular users, although theymos has hinted that this could be implemented in the future if hoarding smerits becomes a bigger issue.