This is not a new phenomenon. Back in 2012, Pew’s surveys showed Mr. Obama ahead by 34 to 25 among voters from 2008. If you have a really long memory, you might even remember controversy about polls that showed people recalled voting for George W. Bush over Al Gore in 2000 by a comfortable margin in 2004 polls, even though Mr. Gore won the popular vote. But it’s not perfectly consistent, either: With Mr. Bush’s popularity flagging in 2007 and 2008, more polls started showing that voters recalled voting for John Kerry in 2004.

With these figures in mind, the U.S.C./LAT poll’s decision to weight its sample to 27 percent for Mr. Obama and 25 percent for Mr. Romney is quite risky. If the panelists, like those in other surveys, are likelier to recall voting for the winner (Mr. Obama), then the poll is unintentionally giving extra weight to Republican voters. Or you can imagine a counterfactual: If the poll were weighted to 33 percent for Obama and 25 percent for Romney (per the NYT/CBS numbers), then Mrs. Clinton would hold a more comfortable lead.

Now, the U.S.C./LAT survey is so different from other polls that it’s possible that its lean toward the G.O.P. isn’t because of its use of self-reported past voting. It’s an online panel, not a live-interview survey, so perhaps the bias toward the winner in a past election is less acute in that setting. In 2012, the RAND panel took a similar approach and didn’t seem to have the same type of bias. Indeed, the U.S.C./LAT poll’s methodology report defends the decision by citing a RAND study of 2008 panelists:

In our preparation of the RAND 2012 Continuous Presidential Election Poll, we found that members of RAND’s American Life Panel were very accurate in their reporting of their voting four years earlier: More than 90% of the reports in 2012 about voting in 2008 coincided with their reports immediately after the 2008 election, for those panel members that participated in both surveys (Gutsche et al., 2014; Kapteyn et al., 2012).

The 90 percent accuracy doesn’t necessarily indicate that the measure is unbiased (if the 10 percent of switches were all people going from John McCain to “can’t remember,” then weighting to the 2008 result would be very problematic). Nor would it prove that new panelists would be as accurate at recalling their vote as longtime panelists. It’s even possible that the past-vote bias may be more acute this year, with enthusiasm for Mr. Obama at fairly high levels compared with 2012. But it does at least raise the possibility that the measure might be more useful in this format than in a typical telephone poll.

There are many other things that could be causing the difference between the U.S.C./LAT results and other surveys — like its unusual probabilistic measure of vote support (voters indicate how likely they are to vote for a candidate on a scale from 0 to 100) or the difficulties of recruiting and maintaining a panel.

No matter the cause, the U.S.C./LAT panel is still useful. Since it recontacts the same voters, it’s easier to distinguish actual shifts among voters from changes in who is responding to a poll. So while the poll may show Mrs. Clinton up by only 1 point, the trend line — an eight-point shift from Mr. Trump’s seven-point lead after the convention — is still very telling.