You are correct that an "eat less fat" mantra really has no science
behind it, in that each fatty acid has unique effects on the body.  One
thing Americans never hear is that there are plenty of very health
Asians with high cholesterol levels and diets very high in saturated
fatty acids (from coconut), and yet they are also very healthy - hardly
any heart disease, cancer, diabetes, etc.  I've read the studies, and
I've also read the WHO statistics, and it's quite clear that the very
broad claims of nutritionists probably do more harm than good, though
few people seem to listen to them anyway.  However, lowering fat is a
good idea if your diet is very high in polyunsaturated fatty acids.
Because of lipid peroxidation (free radical damage) from these fat
sources, your body goes into a chronic  inflammatory process, resulting
in weight gain and lowering cholesterol (due to liver dysregulation).
For example: "Most of the corn and soybeans used to fatten cows, pigs,
and chickens..." Newsday newspaper, May 4, 2005.  It is well known in
agricultural circles that a highly saturated fat source, like coconut
oil, makes people and animals thin, whereas the highly polyunsaturated
fat makes people and animals fat.  However, because these farmers want
to fatten up their animals and sell them to you as meat, there is no
incentive to change anything, and the nutritionists are clueless -
basically, I do the opposite of what the nutrition textbooks say
(because the mechanism is based on free radical biochemistry, and
nutritionists are lucky if they can pass biochemistry with a C, so
don't expect any good information from them).  It's all changed
recently, as Dr. Richard Stein of the AHA recently said about
understanding that only oxidized choelsterol is dangerous, so worrying
about cholesterol levels is meaningless.  If your cholesterol is
oxidized it probably doesn't matter what the level is, what the HDL to
LDL ratio is, or any other such data.  If your cholesterol is not
oxidized, only a level way over 220 indicates that your body may be
having some sort of major problem (because cholesterol is an
anti-stress substance, and so a great deal of production means there is
some sort of stressor, and then other tests would likely reveal what
the problem is, though most, if not all, "chronic diseases" appears
from the evidence to begin as free radical damage to cells and cellular
constituents).

What caused the great variations?
When this question remains unanswered the study was not controlled well.
I suppose they would have mentioned it on medline in the article if they
knew what caused them (because then they could be compensated for).