Why was I paid less than stated on an invitation & put into another survey?

This typically occurs when you were invited to a survey that may have closed, reached its quota for people matching your profile, or was disqualified shortly after you started it and you were routed /invited/ to another survey that was more suitable for you. You were informed about incentives and asked to accept or decline before the start. Since you accepted it, after completing you were rewarded properly to what was stated in the description for that particular survey.