Actually I have definitely seen this happening in quite a few families including our own when I was growing up. I do believe ALL parents think they treat their children equally but they don’t. I think many siblings believe they were all treated the same but I doubt very much that’s true.
The first child is typically (nearly always) a delight to new parents: they actually created another human being. They take copious photos, show their achievement off to family, friends and strangers. Enjoy buying those first toys
The next child is usually a ‘gift’ to the one who would have been an only child. BUT the idea of making a ‘new’ life is wearing thin, even if just a little bit. Parents are still proud but it isn’t the same. It’s a little like the thrill of that first rollercoaster ride again and realizing that you know when the fall will happen this time (or think you do) and when the next turn is. You’re also already prepared with highchair, little dishes, diapers, vitamins etc. You’re getting into the groove.
The middle child is great, and you now are pretty experienced. There are plenty of already purchased t-shirts, nighties, diapers and all the other purchased strollers, car seats etc. that you stored because by now you, as a parent, are not naive.
The youngest is born and you know this is the last one. The older ones are no longer the cute babes and you decided to enjoy this one to its fullest. The hand-me-downs are fine but you also want to enjoy buying for the last time too. This one will be (usually) the last to leave the nest so this is the one who will keep you warm and feeling young for a while longer.
Birth order DOES affect if you are surrounded by other siblings that are close in age and usually they are. Late-life children are a surprise to everyone and are often FAR behind in age by 10-15 years from the oldest or even the next up. They are NOT then actually ‘siblings’ except genetically because the others are often already out on their own so that late-in-life child is more an ‘only’ child.
Most people put blinders on when it comes to this issue. It was once said that the oldest sibling got all the good genes which is stupid really…..it wasn’t the ‘genes’ it was the time a parent or both could give along with the parent’s feeling their child was a badge of success (which is also stupid).
NOTE: I was 15 when I was sent away by my parents. For many years that made me angry, but only for a little while. Now I’m glad they did.