Adam is 100% right on this point, but I've had the benefit (misfortune?
) of being asked this exact question previously, so let me expand a bit on what he said. What you have found is a feature of the English language that Law Services is exploiting. I had planned to add a note about this in the next edition of our explanations, so I’m happy to address it here.
First, from Law Services’ viewpoint, the “both” interpretation is clearly what they intended, because without this interpretation, some of the “correct” answers would no longer be provably correct. For example, in question #13, answer choice (B) would suddenly be a possibility if the first site could date from the 10th century. However, knowing that Law Services intended this interpretation doesn’t help us during a game because obviously we don’t know the answers to the game prior to doing it! I only mention it as being indicative of what they ultimately wanted the interpretation to be.
Second, and more importantly, from an English standpoint, in most cases, “either…or” means “one or the other.” For example, if we see the rule “either A or B must attend,” this means that at least one of the two must be there; they are not both required to attend. In this particular case, however, we should note that the word “either” is preceded by the conjunction “than” and is used at the end of a comparison. Where this is the case, “either” actually means “both.” For example, “He is taller than either of the other two boys on the team” means that he is the tallest of the three, and “I like History better than either Biology or Physics” means that History is my favorite of the three classes mentioned. In the same sense, if I graduated more recently than either of my brothers, I must be the most recent graduate.
Third, could they have been clearer? No doubt they could have. But, I’ve discovered over time that there are fair number of situations where I wish they had stated it more clearly (like saying “both” in this case). I think that sometimes they aren’t interested in perfect clarity, but rather in whatever they feel is minimally defensible in case someone asks about it.
Finally, the good news is that situations like this have only appeared occasionally on the LSAT, so thankfully you are not likely to have to face an identical situation on your test. And the fact that you are perceptive enough to understand this usage and question it speaks volumes for you future LSAT performance.
I hope the above helps. Thanks and good luck with the studying!