Do Body Fat analyzers tell the truth ??

Interesting discovery today. As it was coming upto the end of the month I thought it was time I calibrated my stats more regularly. So out come the body fat machine. Took a measurement, then realised that I hadn't updated my age - did that and took another measurement - different result ! ..... but nothing changed. So then I asked my son Ross (aged 17) to take a measurement pretending he was me = same result as me. I changed my age to 17 and got the same result as he does = a drop of 7%.

Finally to cap it all I put all my height, age & weight back to the proper settings again but selected female = a massive rise of 11% !

How can this variation of 18% on the same person be true just by changing the settings ... I didn't change .... so what is it measuring ?