My parents said pretty much that to one of my siblings. Their position is that they made enormous, huge sacrifices for their children (which country they lived in, which jobs they did, financial sacrifices to pay for private education and all the rest of it, on top of carrying the emotional toll (as parents) for their children’s “failures” in life (eg divorces, hospitalization for this or that, redundancies and work stresses etc). They planned to do all that for their children because we were their children and they chose to have us.
By the time grandchildren started appearing, they were older and wiser and more comfortably off. They felt a duty towards these grandchildren, their own children’s doings. My parents wanted to ensure they have the start in life my parents could afford them if we parents couldn’t manage it ourselves but more loosely, as they’re only grandparents. I think in the early years they saw those grandchildren as their responsibility in the sense that these babies were born to parents who they were responsible for, so if the parents were duds/married duds, it was their fault/responsibility. That tailed off over time as we all proved to be good parents by their reckoning, but it definitely started off that way.
If you ask them today, they’d say they love and enjoy their grandchildren much more than they do their children because they can enjoy their grandchildren without the burden of responsibility for them, and they can enjoy their childhoods / children-ness without having to worry about things like teaching manners and correcting behaviour and ensuring homework is done etc. That’s for parents to worry about.
When the step-grandchildren appeared, they were treated like neighbours’ children: politely, kindly, equally on a human level (feed one, feed ‘em all etc), but that’s it. They don’t feel, don’t want to feel any sense of duty or responsibility towards them. They have grandparents and parents of their own. They’re members of other people’s families, it’s not for them or on them
to take them on as one of theirs. Also, how could they? If even my sibling has to defer to the bio parents, what rights to my parents have? The corollary to no rights is no responsibilities. So, they’re nice and polite and kind and decent, but absolutely nothing like as good as they are to their own grandchildren. These step-grandchildren aren’t their grandchildren. They’re somebody’s else’s. If my sibling-in-law tried to “make” them treat them as such I think my parents would just laugh.
It would be different if those step-grandchildren had nobody in their life by my sibling and their spouse, and my sibling took them on as their children in all respects. In that case, my parents would absolutely, definitely treat them the same - probably better, knowing them, to compensate for the unfortunate start in life they’d have had compared to their own grandchildren.