If we're at LR2, and each level is effectively unaware of the levels above it, how do we know what LR3/4/5/6/7/8/9 are or might be?

Or do you mean that a civilization at a particular level will always be unaware of civilizations above? That doesn't seem to make sense either; I see no reason why a LR4 civ couldn't have knowledge of a LR5 civ, for example.