Original celtic bards were actually historians. All of celtic tradition was oral, so someone had to remeber it and pass it on. The poetry and music were only added to make it easier to remember. We can imagine the D&D Bard as a kind of wandering historian-in-training. He already knows some of the songs/stories, but has a long way to go - as he gains experience, he learns more and more of them. But he isn't performing what we would call a scientific reaserch - he is wandering the land, collecting pieces of lore and legend.
So, for example, he may know that a vampire can be killed by putting a stake through his heart, beacause he knows "The Song of Barabarus, the Mighty Vampire Slayer", but he doesn't need to know that the same vampire is weak to light, since it isn't mentioned in this song. He would need to know "The Legend of Saint Carnus and the Dark Beast" - which, while technically covering the same topic, is about a completly different person from completly different land, and thus not a part of "a pack". For someone which aproperiate scholary knowledge, this two facts are tied together as a part of vampire lore. For bards, those are separate things, tied to specific places, clans and stories.