Since being accepted on 10th January this year I have had 277 pics accepted on SS -a 5 week period. It took me nearly 12 months to reach that number on DT!  I am over the moon as SS likes my designs which DT hates.   Working very hard to reach 1000.

Wow, SS, cool! It's much more excited if the sale is high also. 

On my side it is apprx:
DT 1
SS 15

What does your DT portfolio comprise of Vannphoto?

FYI - If you click on any of the agency icons at the bottom of the poster's entry you will go directly to their port(s). Many enable that feature here (some do not) and it is a quick way to see what types of images they have where.

Shutterstock is definitely top earner for me, with similar size folios on many agencies.
After I uploaded my first around 300 files every month yielded around 50$ (with very little variation).
For some months with no further uploads these earnings remained stable.
At a later stage I uploaded around 300 more (double the files) over a couple weeks, then the earnings went up to 100$ (double the earnings) and remained so for months until I
recently uploaded 150 more and the earnings grew proportionally again, with very little variation.
It seems strange these figures, and this is over a 2 year period total.
The monthly variation is never beyond 10%, so I can predict quite accurately the earnings at the end of each month.
So you have x files you make x money, you have 2x files you make 2x money, you have 3x files you make 3x money...?
This doesnt happen with any other site I'm on.
Some sites can make 5$ one month and 50$ the next, a variation of 1000%
A strange recurring coincidence? Is it suspicious? What are your experiences?

What is the reason for SS reported sales patterns being so very dramatically different from all the others?

actually if you look closely SS is the more consistent (growth too), so I believe you should suspect all the other