Hi,
I've googled and seen a couple of conversations on this, but the go
way back to August '06.
I've posted similar to this elsewhere. Not crossposted.

last week, I bought a Tanita TBF 602 {for 65 Euro} and have been
having some fun experimenting with it.
This models allows for 2 people to use it (not simultaneously) without
re-setting the details (height & gender) and there is a booklet which
provides guidelines as to ideal fat/age ratio.

Firstly, I do gardening/landscaping for a living. I work long days and
everything nearly always has to have been done yesterday.
I have a 20 year old part time footballer for a colleague and he finds
it can often be tough going. So you get the picture. I should be fit.

The maximum % fat it suggests for guys my age is 23%.
So, imagine my surprise to find that I am supposedly 35.5.
My wife's details however, far kinder than my own, seemed more
probable.

Then I noticed that it changes ... not only because of time of day,
and not because of recently drinking water but according to what I'm
wearing and what I might have in my pockets.
But did change after gorging on delicious, traditional, homemade,
Christmas pudding.

So, readings, when naked, or when wearing, say, only cotton, or mixed
fabrics, with or without wallet and paper tissues vary widely
regardless of time of day, exercise done, or intake of foo or liquids.

Just for fun, we've a large black cat who, according to Mrs a.,
doesn't eat a lot, but he's still got quite a belly and weighs in at
nearly 7.5kgs... and the large volumes of cat food I hauled home not
so long ago seems nearly gone. Urm...
Anyhow, Mrs a went on the scales earlier today and was pleased to not
her weight had dropped, but not so pleased to see the %fat had shot up
several points.
Undeterred, she repeated the exercise with big fat black cat in her
arms and the % fat shot up by another 9.5 points %.

So, as you can see, what you wear, or are holding (and maybe what you
may be physically in contact with) can make a difference.

And now another possibility;
After I did it today, I got off and suddenly realised I'd forgotten to
check my weight. So, I tapped it on again and stepped on, but on
seeing my weight, I suddenly saw my %fat had dropped suddenly to
19%... from less than a minute earlier, with no change in clothing (or
its contents) and no cat in sight.
However, my wife had placed it near an electrical power socket/ plug-
socket and just behind me, (on a tiled floor) there were the speakers
from the music centre.
So, either its proximity to a plug socket, or relates to the output
from the speakers... or I'm an alien.

BTW, I see today that our local Aldi store are now selling such
scales, (I don't know which) except that they not only measure weight
and body fat, but also water retention levels and muscle mass.
So, that's one week after I've bought mine, it does 2 other readings
of interest and all for a mere 23.99 Euro
GRRRRRRRRRRRRRRRRR