First of all, all you're showing is the literal amount of damage they're capable of. This doesn't say much about how good the pokemon actually are.
Second, I have never seen a full SpD celebi, meloetta, dnite, or togekisss. Azelf is rarely used in OU and would never run full health. So the only pokes there that like are real counters to landy are cress, gyara, and the blobs (also, why did you calc fblast vs. gyara and cress instead of hp ice--does less).
On the other hand, every single one of the pokes that counter pory-z is viable in OU and commonly runs a SpD set (maybe excepting zone, but bulky with a lot of hp is common). So really, the number of actual pokemon you will ever run into that counter each sweeper is 6 for pory-z and 4 for zone.
Additionally, not every team tries to beat an RP sweeper by fully countering it. Instead, they'll throw out a poke that can take a hit and KO back. And there are a lot more pokemon that can take one hit from pory than from landy--just about any steel in OU for example.
Lastly, pory is just so much more easily defeated in other ways--priority, residual damage, not having good defenses with which to set up, etc.
In terms of the number for SpA, yeah, pory can get higher with DL boost. But it's way easier to beat than landy.
