I was mortified/horrified/surprised when my 5 year old (middle child - ironically the one we always thought of as the skinniest) was flagged up as overweight. Initial reaction was anger and denial, but I got a grip and took it further. I spoke to the health team and asked for some advice and info (I'm very 'into' child nutrition, we do all the 'right things', home cooking/healthy packed lunches etc), as I thought there was nothing I was doing wrong and must be a mistake, but I wanted to just make sure there wasn't something I'd missed.
They sent me some very interesting info about portion sizes (he was having really healthy food but too much). For example, 2 weetabix is enough for a child of his age, they don't need toast and fruit/yoghurt aswell, 2-3 tblsp pasta etc. Instead of dismissing health teams (which seems a favourite pastime on mn), we should be using their resources and advice - they have no personal agenda with your child, and no reason to define a child as overweight if the figures don't prove it so.
It's very easy to see eating and a big appetite as something to be celebrated - lord knows, there are enough fussy eaters that it can be seem as a sign of good parenting by some if you have a 'good eater' - but we need to be aware of how much and what we're feeding our kids. These weighing occasions provide a reality check and showed me how we'd slipped into bad habits over the summer holidays which had continued (too much snacking etc). Also, his older brother is very tall and skinny, and obviously is built to burn whatever he eats without putting on weight. We needed to recognise that they are built differently and not assume their body shape/make up was the same, and that they shouldn't necessarily have the same portion size.
Looking at him with fresh eyes we could see he had a bit of a tum (we just thought it was still a baby podgy tummy), and was no longer a skinny little thing. He was never aware of any of this, but as his parents it's our job to monitor and act on changing health needs - being overweight is one of these things.
I see many overweight children at my children's school, and as a person who's struggled with weight myself I feel desperately sorry that there doesn't appear to be a grown up taking control and helping them to slowly and sensibly change their eating and form healthy habits going forward.
These tests are useful - if you have an issue it will let you know, if you don't then no further action required. When it comes to your child's health, knowledge is power.