It's nice to be author of a conspiracy theory, so I thought I would share this with you: 
I've noticed for some time that the iStock exclusive tally in the Microstock Poll Results bar is almost always approximately equal to the sum of the top four independent sites. Perhaps iStock caps its exclusives at (= Top 4).
I guess it would be possible to tweak the earnings average to keep it at a desired level, and presumably the people submitting to the poll are generally the same bunch each month so their decline (or rise??) in earnings will be pretty consistent.

Great! This conspiracy makes a lot of sense.

Now I know why we have to wait until the 20th of following month for results: they need the results from other sites to tweak their own.

I think you're onto somethig.

I would consider your theory but with one change. I think they cap the number of downloads, not the earnings. I'll have to go back to my log to check that I have been between 67 and 69 downloads every month for many months. Somebody must have bought a couple of pricey images as my earnings were almost double in the last payout - but the downloads count was incredibly still consistent. Of course, capping the downloads count will have the effect of also controlling the earnings as there will be some general averaging among sales. Has anybody else checked there downloads from month to month? My port is not seasonal material. How about somebody with heavy seasonal material - do you have steady download counts or does it follow a seasonal cycle?

I think there are six or even fewer exclusives reporting most months. Some exclusives report and also report their Alamy RM sales: this pushes their sales to iS-indie (just because of the way the poll is set up). I report my iS sales, but not my Alamy RM sales, for that reason.

Downloads by month:
2014-01   59
2014-02   59
2014-03   57
2014-04   64
2014-05   55
2014-06   57
2014-07   52
2014-08   49
2014-09   47
2014-10   45
2014-11   56
2014-12   39
2015-01   24
2015-02   19
2015-03   42
2015-04   28
2015-05   47
2015-06   29
2015-07   24
2015-08   23
2015-09   31
2015-10   26
2015-11   35
2015-12   21
2016-01   20
2016-02   31
2016-03   25
2016-04   21
2016-05   20
2016-06   19
2016-07   12
2016-08   17
2016-09   8
2016-10   9
2016-11   10
2016-12   6
2017-01   48
2017-02   73
2017-03   69
2017-04   83
2017-05   70
2017-06   75
2017-07   73
2017-08   73
2017-09   74
2017-10   66
2017-11   73
2017-12   69
2018-01   60
2018-02   66
2018-03   65
2018-04   72
2018-05   65
2018-06   66
2018-07   65
2018-08   66

Be afraid, even the ears have walls ;>)