There are two kind of assumptions people make when discussing about things with others: either they assume the person doesn't know about the thing they are talking about or they think they already knew about it.
I hear a lot of the former one in my job. People come to me to tell about something they have just discovered, or thought about. Rarely do they tell me about something completely new. Of course it's my job to keep up with the matters so not many things in our fields come to me as a surprise. It's still assuring that others find those topics important, worth to bringing up to me.
Assuming the other way around is more dangerous. Even though I "know it all", I really don't always. If people keep assuming I already know and therefore don't bother bringing those topics up with me I will never know.
So it's better to appreciate whenever somebody brings up such topics. Even if I knew beforehand about it, it gives better incentive for people to keep telling me about things so I don't have to worry too much about missing something important.