Sometimes you get the follow-up because there were technical problems with a few questions with the original survey. The other day, I did a survey where the radio buttons were clearly messed up on one question. Then today, the first survey that came up was the corrected version of that single question from that survey so the radio buttons worked right. So the same credit (25 Jet Blue miles in this case) for literally three mouse clicks as for a survey I could be spending 8-10 minutes on.