Hierarchical grouping in Views?
I found the answer to my question by tinkering. Like most solutions to my aggravating questions, it's stupidly simple.

I checked "remove tags from rendered output".

Upon inspection of the HTML page source, I found that the attribute content="[long, granular-to-the-millisecond datetime]" (e.g. content="2011-08-01T00:00-04:00") was plopped inside a &lt;span&gt; tag that was wrapped around the rendered output. So even though it looked like it was being grouped only by the 4-digit year value, it was actually being grouped by this whole, long value:

&lt;span class ="date-display-single" content="[datetime]" datatype="xsd:dateTime" property="dc:date"&gt;[Year]&lt;/span&gt;


Since each of these contained a totally unique datetime value, it appeared not to be grouping by year at all.