I am not a CDC but I can have a stab at this and I am sure that anyone who knows better will correct me ....
My take is that this, and any diet, works based on the calorie defecit you have .... SS you are at a calorie defecit of 2000+ calories. The body finds what it needs from elsewhere - glycogen, body fat.
The more calories you take in, the smaller the defecit, so the less the body has to find elsewhere, so the less you lose - hence the slowing of the weight losses as you step up the levels.
If you are SSing and picking then in the same way you are reducing your calorie defecit, hence slowing your losses.
Simple maths says that if you step upto 790 and still pick, then being on 790 you are on a smaller defecit already, so continuing to pick will just reduce that smaller defecit, and reduce your losses still further.
It could be, however, that if you step up to 790 you may not need to pick at all. I have seen it said many times in this forum that being on 790 properly only slows your losses by a couple of pounds a month.
The bottom line is that picking at any level will have an effect, and slow your losses down, simply by the fact that you are taking in more calories, carbs, etc
Steve