Something to keep in mind is that there is a constant battle between the spam-filters and the spammers. Spam-filters are always upgrading themselves based on stuff behind the scenes as well as what it reported as spam by all their users. Spammers on the other hand are constantly updating their code to get around the constantly updated filters. Like any long-running battle, it ebbs and tides back and forth between the two. For a while the filters will be stronger than most spamming code, then the spammers will figure a way around, and then the filters will update themselves to block the new spamming methods.
It's perfectly normal to see spurts in spam from time to time as this battle goes back and forth with spammers and filters taking turns being better than the other.
What still baffles me is that there are enough people who actually respond to the spam that it is financially worthwhile for the spammers to keep up this battle.