If a datepicker is set to show the year dropdown with a yearRange setting not including today, it still renders the current month when it first comes up. For an example, see http://jsfiddle.net/2ScGt/. The yearRange is set to "2009:2010". It's currently October 2011 (as of writing), so when the datepicker comes up it's showing October with 2009 selected in the year dropdown. But it's rendering October 2011; 10/1 is on a Saturday instead of a Thursday as it was in 2009. Clicking a day in the picker enters that day in 2011 in the textbox. So clicking on the 1 in the month while the datepicker says it's October 2009 puts 10/1/2011 in the textbox.
I think this is related to http://bugs.jqueryui.com/ticket/4641, but 4641 doesn't mention the rendering specifically.