I guess you didn’t know what you were agreeing to (or were agreeing to semantics, not substance). Anyhow, it’s not a mathematical truism, it’s an anecdotally observed phenomenon.
Your theory is mathematically incorrect, so it simply can’t be true.
You can either accept that your observation, that the gap is the same, is incorrect, and others’ observation that it has risen is correct, or, I guess, you can (continue to?) argue that Hashem made a nes and suspended the laws of math, (as He did in Noach’s teivah), and would continue to do so if only we pooled the dates so that boys are told who to date and all girls get a fair shot. I guess on your (real?) planet, Hashem is punishing us, for not forcing everyone to date who we want them to, by suddenly following normal rules of math.