I agree that for one reason or the other, your mother is experiencing inner turmoil. Since that (inner turmoil) is what gets just about all of us out of the WT society, even the extremely logical of us, I'd just let it lie (pun intended).
Another reason she might be doing this, slightly less hypocritical (or at least justified in her mind)? Perhaps she is thinking that your son will be getting gifts at this time of year no matter what she does, and therefore she wants to make sure that she, as his grandma, does not fade in importance in your sons mind, comparitive to the others who will be giving him gifts.
Plus, being his birthday and all, she's already thinking of him