It's just an idiom and not meant to be taken so literally. Basically if you're getting a 100x improvement, you're jumping from one paradigm to another. And those are probably already solved algorithms / invented datastructures. If you're doing something novel and "clever" to speed up your algorithm for your circumstances, it's probably going to be under a 5x improvement.

Or to continue the SAT analogy, if you're doubling your SAT score (e.g. 700 to 1400) you're getting smarter. If your 100x'ing your SAT score, the best possible previous score you could have gotten is a 16. Which would mean you got about one and a half questions correct across both the reading and math SAT.