I had never really thought about it to that depth, but definitely do agree with you. Even if your intention is to keep an open mind and capture "all sorts" - if your mind knows that there is some kind of agenda or topic at hand... it will even unconsciously gravitate to that level of focus.

Sort of like when you buy a specific brand of car for the first time. Suddenly, you will start to notice that type of car on the road - making it seem as if there are an unusually high number of them, when in fact nothing at all has changed except the attention your mind gave the matter :D