6 months post op Breast Augmentation, my chest muscles tighten and my implants move laterally. What leads to symmastia? (photos)

Is symmastia primarily caused by surgeon error, anatomy or post-operative care? Also, are most board-certified BA surgeons trained to revise symmastia? Or only specialized reconstructive surgeons? Lastly, what is the average cost of [just] symmastia revision in the Pacific Northwest? I have been doing some research about symmastia and I've noticed a very broad range of cases.. do I fall somewhere on the spectrum? This topic has not been discussed with my surgeon, but I'd like to know what you guys think before I bother him. Standing upright, my chest muscles tighten, my implants move laterally and the contour of my cleavage is more defined (slight tenting). However, when I'm seated or hunched over, muscles relax and implants would slide medially and appear to be continuous. BA 400cc mod silicone under muscle- sternum skin raised, bras cause midline joining and diminish cleavage