I think the problem with parents comparing in class is very much that classes are different, but parents don't necessarily realise this, nor are happy to hear if their class is below average.
When dd1 was in infant school, I would go in and help, and sometimes I was given a maths group to work on. I worked on it with the top two tables in her form, sometimes I then was asked to go to the other class, and only the top table would be doing that work-and often struggled with it too.
At the time I didn't really think about it, assumed that it was just the teacher's different working patterns. But when they went to juniors (double the size) they were setted in maths. From dd1's old form, all the top table and a couple of the second table were in the top set. From the other form there was one from the top table who was during the year moved up into the top set.
Now I know that there was a parent from the other form, who when she discovered that her ds wasn't in the top set (which took a bit because she assumed they were, had an awkward conversation where this was very clear) stormed in because her ds had been on the top table in infants, therefore should be in the top set now.