It may be just that they're limiting the number of invites/completes for more active members. I'm sure survey data of active respondents is likely to look different to data given by less active respondents. Maybe they're just trying to get a balance of respondents.

I'm just fed up of being offered the profilers right now which I get every day. I'm lucky if I get two survey opportunities a day now. In recent months I've consistently had at least double that for points-earning surveys.

e-rewards are owned by Research Now, who are a pretty stable and major player in the research industry.