If I remember correctly, it has something to do with the diaphragm still closing down to the working aperture while the shutter is already open. Some users commented at the time that not all A-lenses suffered from this, so it may be nice to have the option of EFCS. I used a few A-lenses with the LA-EA3 on my A7R2, notably the 4/35-70mm and 4.5/100-200mm, always with EFCS, and I never had problems.