Roughly put, learning is really just a growth in awareness. The transition from not knowing to knowing is part of it, but that's really too simple because it misses all the degrees of knowing and not knowing. One can't ever really, truly understand something any more than a shrub can stay trimmed. There's always growth or decay, changing contexts or conditions. Understanding is the same way. It's fluid.
Yes, this sounds silly and esoteric, but think about it. While morsels of information -- math theorems, for example -- may not change, the context in which students use them do change. Which in turn changes how we consider and use that morsel.