Cognition: Is it common for people with dementia to be really observant and then really out of it?

Hi
My gran is confused a lot of the time but then she says something that is very astute. For example she says she will not ask her son a particular question because he skirted around it the last time she asked.
She sometimes observes things I do not notice while other times she is unable to follow what’s going on.
I am glad that she is very perceptive sometimes but I get very confused because sometimes she is oblivious.
She is unable to live independently.
Is it common for people with dementia to be really observant and then really out of it?
thanks.