OK sorry--user error it seems. The task 2 work allotment for my resource was set to 160 hours (the full 4 week duration), rather than 12 hours. I changed that, and it resolved the issue.

Thanks very much for your time. This was still a good learning exercise for me. We can consider this resolved.

edit: actually, this is related to an issue I've been experiencing with Project 2013. In the Task Form, I can set task type to Fixed Duration, enter a duration for my task, then enter a different duration in the "Work" column for my resource. When I press OK to accept these changes, the duration in my "Work" column resets to match what I entered for task duration. This was the reason for this issue (my carelessness persisted it). It also slows me down when entering such information, as I have to do it twice. Is this a known issue, or am I doing something wrong?