Heather, that's the million dollar question and I just don't know the answer!
The BodyGem test is supposed to be fairly accurate - I was trained by the manufacturer when I started to work at Ballys and they have all sorts of studies to back it up. And no, you're not misunderstanding BMR/RMR either (for our purposes they're the same). All these things are pretty accurate for 'normal' (never obese) persons.
But all the rules change once we lose a lot of weight. We - the 'reduced obese' - live in an alternative universe.
All I know for sure is that I can't eat nearly as many calories as I should be able to and maintain my weight according to metabolic calculators, the BodyGem and probably any other measure you can think of. With a RMR of 1600 and the amount of exercise I do, I should easily be able to maintain at over 2000 calories per day. But when I eat that much, I gain. Grrrr ...
Why? I read and research this all the time to try to figure out what's going on and my belief is that it's biochemical and hormonal. My body as a 'reduced obese' person is very, very different from someone of the same age and size who never was overweight. The weight I'm maintaining is not where my body wants to be; it wants to re-establish its equlibrium back around 250 pounds. So it's sending out all sorts of hormonal signals to get me to eat and get my body fat back to where it used to be by conserving energy and calories. It makes sense from an evolutionary perspective, of course, but sucks in real life.
There's a reason why 95% of dieters who lose weight regain it all again and it's not just a lack of willpower or loss of focus. We're contending with fairly powerful biochemical messages to eat more and regain weight. It's NOT a level playing field and we're NOT playing by the same rules as 'normal' people. Our bodies want to be fat again - it's that simple.
One one hand, this is a fairly depressing message for those of us who were/are obese and I don't mean to bum anyone out or worse, have them give up entirely. But on the other hand, this is REALITY. Better to know what we're up against and make a plan to deal than to be unprepared and end up regaining.
I guess I deal with it by not comparing myself to anyone else. It doesn't matter to me what I should be able to eat and what other people do - it's just not relevant to me. I've already defied a lot of odds by losing down to a normal BMI at age 46, after being hypothyroid and obese my whole life. I suppose that I've defied even more odds by keeping the weight off for more than four years now. But it hasn't been particularly easy or natural for me. It all boils down to THINKING. I eat with my head and not my stomach, which means that I ignore the cues my body is sending out to eat, eat, eat. The day I rely on intuitive eating is the day I start regaining weight because my body intuitively wants me to be fat again. I'm completely convinced of it but I refuse to let it happen.
Wow, this really turned into a treatise on something you didn't even ask about - sorry!
But you probably can guess it's something that fascinates me and I'm constantly trying to learn more about.