The "this opportunity has expired" responses never seem to pay out anything, while the "Your e-Rewards account has been credited the partial credit amount" responses credit as usual.

My recent experience has been that one of those response types tends to be followed by many of the same type, even when dumped out in seemingly identical circumstances. (e.g. When having not clicked on anything within the survey at all.) Maybe some really have expired, but then why flash them up at all?