I have a step mother, who's mother would give my brother and I some presents for christmas and a few pound here and there, if she ever did have more set aside for my sister she would give her it in private.
My step mothers, dad was the same he would give my sister her "extra presents when me and my brother were not there.
TBH with you I do feel grandparents do feel more connected with there own and do want to treat them that extra bit special, and I have no quams with this, and do not feel they wronged me in any way shape or form, as they did not need to give me anything.
My kids do not get any attention from their own biological grandmother, and unfortunantly I have no grandmother to offer them, and so my step mother is the only person who fills in that spot, and I am forever greatful for that.
Just remember that "step" grandparents may treat them different, but at the same time if she wants to be there for them even in part, they are lucky.
This is just from what I have read - obviously dont know the whole story, hope this helps. I am greatful as I say to my step grandparents, even if they treated us a little different, they have no obligation to talk to my brother and me atall.