(Sorry in advance for any perceived dehumanization of your grandma)
This is not only a vote for deterministic tendencies, but also a function of architecture.
Assuming she only expressed the non-glitchy neural pathways available to her (since if she accessed a glitchy pathway it would fizzle and not rise to consciousness/motor cortex speech expression), she already has a smaller subset of tree paths available to her to begin with.
Depending on the dementia, she could theoretically have fewer and fewer functional pathways available over time. That makes the same conversation use pathways that for some reason were "glitch-proof".
So, if in the conversation you always prompted her with the same prompts, it would make sense that as the pathways diminish, the same ones would be used over and over again.
The guiding neural pathways responsible for getting consciousness to function must over time be conditioned to select the working pathways, and actively avoid the random intrusion of other networks whose firing would cause novelty and creativity, for fear of hitting a glitchy pathway and fizzling out before consciousness.
So, determinism as a defense mechanism against decay.
Having learned about this game right now, this is my idea to set up a random deck (so not field-tested, and maybe I'm misunderstanding):
Separate into 4 piles of 12 cards, according to suit, face down.
Another person moves the piles about like in 3-card Monty until no one knows where each suit is.
Randomly choose 3 piles to remove 2, 2, and 4 cards. Or always remove 4 from the first, 2 each from the next ones, from left to right or by whatever convention.
Shuffle together remaining piles, deal.