Also, the reason for keeping fat low is to keep you from having excess calories stored as bodyfat.
It's actually fairly difficult for carbs to be stored as body fat, from what I understand, even when you're overeating them.
I think where the fat gain can come from, if I've read correctly, is that basically while having such a large carb intake - your body stops fat oxidation, meaning all dietary fat is then stored as adipose tissue, etc.
All energy must get stored as fat. That applies to carbs too.
For carbs you can use some of it during digestion, because insulin drives the whole body to use up the glucose. The reason why it does so, lies in the fact that it is poisonous for the blood, which can hold only 4-5gms of it. Around 400gm can be stored in the liver and muscles, but most of it did not get depleted except after fast or long heavy exercises.
The rest must get stored as fat. There are no other ways to it.
The problem is not the fat storage.
The only reason people gain weight is because the fat cells are not releasing fatty acids that are stored there. The reason behind this is mostly hormonal. Testosterone and several other hormones drive fat out. Leptin is used to manage the level. But there are some fatty cells that are so damaged that they don't respond to the hormones.
Different fatty cells have different propensity to get damaged. It also depends on people. Most people get fat in the middle some get fat on the lower body others on the upper body. Some cannot get fat at all.
It is not a simple thing. There are several factors that damage the cells. The biggest of them is high omega6 to omega3 ratio. Try to bring it below 4:1 on a daily basis. Anything above is bad. Another big ones are sweeteners, fructose, artificial sweeteners also galactose. These cause glycation, and damage cells receptors. Some others are lectins, but those we don't know much about, except that the WGA (Wheat Germ Agglutinin) is pretty bad.
Whole wheat is quite deadly and is associated with increased BMI and heart disease. See the link
http://rawfoodsos.com/2010/09/02/the-ch ... ase-oh-my/
Then there are ways to force the body to lose fat, which is what low carb + weight lifting does. But it may not be entirely healthy, as it causes a lot of stress on your body. If your adrenals are strong enough handle the stress, then you are able to get to a lower point of weight. But unless the leptin accepts the lower mass, you will forever be struggling with it.
The strategies to increase leptin sensitivity will help a lot.