Yeast spores are everywhere....IMO some of the possible variables that could be contributing factors.... The exposure to outside air before sealing....room temp? Whats the ingredient mix.?(ie.,salt ratio amount of whey per quart)? how well were the fresh ingredients washed...? How long have they been fermenting?
There is obviously something different going on in the middle ferment(given that they were all processed and sealed at the same time/place conditions etc.), and it appears that the first one has more brine covering the solids.
All that being said....many times two identical ferments will behave differently.
While I refrain from opening the vessel once sealed,...if I had the situation your pics show, I'd open and carefully scrape off the kham yeast, add enough additional 2.5% brine to make sure the solids are totally covered and submerged, and reseal....then any future growth that may result from opening the vessel will be on the surface of the brine(or any floaters)....not the solids. Even though the yeast growth is considered harmless, it can ultimately affect the flavor...especially on a long term ferment.
I use cabbage leaves(and sometimes a condiment cup) to keep solids submerged....any yeast growth is normally only on the cabbage, and doesn't get into the brine or any solids.
CM