I think they 'cannot' prognose accurately - because they simply don't know. In Cancerworld there's a saying 'No doctor or medic has your time-of-death certificate in their pocket'.....and I would suspect that is true in a way of all of us. Some people 'hold off death' for much longer than 'generally expected, others succumb faster. It may be a combination of physical state AND 'character' (for want of a better world). When my husband was in late end stage I also asked that question, and was told 'it could be a few days, it could be a few weeks'.....
However, I agree that you do need to tell them 'I want an honest answer' as otherwise they may, indeed, 'soft-soap' you, in an effort to be kind. But I don't think, myself, it's kind, because it can backfire if the person dies faster than we are led to expect. When my father had an operation in his 80s no one 'warned me' he might not come through it, so when he didn't, I was truly shocked and appalled - I had absolutely no expectation that he might die in theatre, but he did! In my case, I think it was a case of 'crossed expectations' - the medics knew perfectly well that a sick man in his 80s might very well not survive surgery, but I, being naïve, had no such expectation. Medics can 'assume' you know things that you don't.
Whatever the situation is now for your poor father, I do hope that the end, if it is now the end, when it does come, comes gently and easily.......for both him, and yourself.
Kindest wishes at such a painful time, Jenny