@snellgrove I think there is positive bias as you said. Also, the people who hang out on these boards tend to be quite invested in education.
I do think though that as things progress to GCSEs people with high achieving kids need to be careful. Too many people seem to bandy their child's predicted/actual grades around even when not really relevant to the discussion at hand. It can put off parents of average or lower achievers from joining in the threads.
e.g. On GCSE results, in my view it is 'good enough' to say 'delighted with DCs results, they beat their predictions in most subjects' rather than adding on they got 8 grade 9s and 2 grade 8s.
Especially when parents add on something like 'they deserve it, they worked really hard', which leaves hanging in the air the thought that maybe kids who got 4-6s didn't work hard...
Of course grades are relevant for various things such as when talking about suitable BTECs/A levels or Unis, including in 'so proud' or 'boasting' threads.