1) Parents treat their children equally

1) Parents treat their children equally...

I disagree. Parents may try to treat their children equally, but in my opinion, often do not succeed. Age is one of the influences on parents, also position within the family (oldest, middle, baby etc). An example would be when there is a small age gap between siblings, like my sister and I, then the younger may get things at the same time as their older sibling - just like my sister and I! She gets what I get (electronics, bed times, movie rating, sleepovers etc).
The first born child gets all the attention until another child comes along .This might be why the eldest can feel that they do not get as much time and attention, because they are used to it ALL!
Personality has something to do with it, some children demand more attention than others.
As the eldest I am the 'experiment' - the first which means parents are not experienced, they learn as the child grows.
Most children think that they get less attention than their siblings and that "it's not fair"! Children exaggerate differences in what they are allowed, children can be prone to bragging to their siblings so that their sibling is jealous of them.
Another influence on the parents would be the amount of time they have for the child. If one of the children have got homework, ASA etc. when the parent has free time, obviously their other children would get the attention at this time.
Although no parent likes to admit that they have favourite children, they might. Maybe some parents don't even like some of their children. This can happen, but people don't like to talk about it.
The amount of attention that is given will vary according to circumstances, if a child is sick, has a disability, is struggling with school etc. then they will usually gets more attention because they need it.

So as you can see, I think that there are very many reasons why parents do not treat their children equally.