I don't know for sure, but the bumping changes in the Altcoin section resulted in less users being exposed to the spam. The spam still exists, it just isn't noticed as much. The reduced activity has, and will always result in less reports, since there's less things to report as you correctly pointed out.

The other thing; I've noticed some users getting a little disgruntled on getting bad reports or them staying unhandled. So, they've reduced their reporting activity, which is a shame. I do get why they would be frustrated by that, but it's not always that the report hasn't been seen it's just been chosen by those that have reviewed it that it was either bad or was better to stay unhandled.

Also, the merit system probably has reduced the amount of users, and also means that a lot of the spammers stay new users, and therefore get picked up by the patrollers. Since there's more patrollers than dedicated moderators usually.