Because memoirs are narrative stories that, though nonfiction, flow much the way a novel does, some agents prefer that you send a query letter for your memoir just as you would for a novel. Other agents treat memoirs like other nonfiction submissions and prefer that you send a book proposal up front.
This means you’ll need to pay special attention to the guidelines of any given agent before you submit. It also means you’ll likely need to do double duty in preparing your submissions materials before you send out anything at all. You don’t want to find yourself in a situation where an agent requests materials you don’t have at the ready. This is especially tricky when it comes to the manuscript itself. Agents who require queries will expect the full manuscript to already be completed (and revised, and polished). The agents who require proposals, on the other hand, may be expecting to have input into the shape of your story before it’s written.
Frustrating? Yes. Avoidable? No. Your best course of action is to err on the side of being as prepared as you can, and trust that all your hard work will pay off later.