Obesity is a run away process in which adipose fills and becomes insulin resistant and starts spilling some of the fat it has stored, the pancreas responds to fat + glucose in circulation at the same time by making even more insulin, so more energy is stuffed into adipose until it becomes even more insulin resistant, etc
At the same time insulin is stopping cells from moving fatty acids rapidly (via the carnitine shuttle) into mitochondria to be burned, and instead they have to take a slower route (peroxisomal cleavage). So at this point any fatty acids spilling from fat cells unable to hold any more are not burned, but bundled into triglycerides and stuffed into LDL particles (yes @DaveKeto VLDLs really) which is why a diabetic has incredible levels of triglycerides. Mine were 1111 mg/dl.
That goes on for up to a decade, the pancreas making more insulin, the body desperately trying to make new adipose cells, and grow existing adipose cells which are becoming filled to bursting.
Then we go low carb and all of a sudden there is a LOT less glucose and the pancreas gets a break and insulin starts dropping and all of a sudden all your cells can burn fat at pace (the inhibition of the carnitine shuttle is relaxed) and as that insulin comes down your fat cells finally can start delivering energy into circulation - which is now being burned.
And with a great whoosh all that pent up energy is released - and THIS is why diabetics who go from a high carb to a low carb diet lose a lot of weight very quickly … and why it’s all fat and not lean tissue.
The real question is why do they stop losing weight? Why do they plateau with still some weight to lose before they are at an 8% body fat or whatever their ideal weight loss is.
The answer appears to be (this is my hypothesis that could well be wrong) that multiple tissues have independent insulin resistance levels and the one that drives the most secretion of insulin determines the amount of insulin secreted and that sets the insulin sensitivity of the rest of the tissues to perform their function. So even tho your adipose tissue no longer needs to be insulin resistant to hold onto more energy, some other tissue is holding insulin higher than your adipose tissue needs to hold onto energy.
So the simple way to explain this is that we plateau at our underlying level of insulin resistance after the Adipose tissue no longer gets a vote.
Why do some people plateau higher than others? They have higher fasted insulin. It might be tissue in an organ like the liver, or the alpha cells of the pancreas that’s requiring insulin to stay elevated, maybe it’s tissue in the hypothalamus that drives our energy signalling, maybe it’s muscle tissue, maybe a multiple.
The important thing is that Insulin exposure begets insulin resistance which begets even more insulin exposure … etc.
This is why we encourage people who plateau to keep calm and keto on because the longer they are at a lower level of insulin thanks to a ketogenic diet, the more likely the tissue keeping insulin higher becomes more insulin sensitive.
Sorry that’s a long complex answer to a simple question. The simple answer is people who plateau probably have fasting insulin over 14 mIU/l, and each us get there a different way.