This is because of the split into science & culture tech tree. In earlier civs the tech tree was a sort of "guideline" that always dangled the next "carrot" in form of the next tech before your nose. It had 20+ techs per era & told you "you are here right now" in a pretty specific manner. And of course, because you were always developing one tech at a time, you got the feeling: Ok, I'm now in era xy. Now, you have two trees that naturally have half as many techs. You can potentially go animal husbandry -> archery -> horseback riding -> knights & bamm!, you are in the middle ages. You develop two "techs" in parallel & you can potentially be in different eras in science and culture.