Why did Zorin go straight to 15 from 12.4?

I don't know why the Zorin devs skipped versions 13 and 14. Like, yeah, they seemed to jump from version 12.4 to version 15 for some reason.

This was done to place the numerical ordering back on track, since there had been two Special Editions of Zorin OS that did not use the numerical ordering in the past. Including them back in covers 13 and 14, putting the edition of Zorin OS following 12 to 15.

I'm guessing Core and Ultimate?

I should have said versions, not editions, I think.
No, there was a "Zorin OS Limited Edition" for one, I do not remember the other.

Interesting... I was going to ask if that was for a special cause but I don't know if you know about it. The Zorin team should do this again.

From the time I was a moderator, I started with ZorinOS 4, then 5, then 6 was the first LTS release. 7 and 8 were development releases, in much the same way Ubuntu releases 6 monthly development releases. Zorin 9 was the next long-term release. I really liked 10 and 11, also development releases, then 12 was the next LTS based on Ubuntu 16.04. The decision was then not to do any more development releases and so Zorin 15 was the next LTS, based on Ubuntu 18.04. So you should now notice a pattern - every 3rd release (increment of 3 i.e., 6, 9, 12) was LTS. After 15, no more development releases. So Zorin 16, based on Ubuntu 20.04 became the next LTS iteration, logically following on from Zorin 15, so LTS releases for Zorin will now in future go up in ones, 17 will be based on the LTS of Ubuntu 22.04. There never was 13 or 14 as far as I can remember. They did used to produce Business and Gaming Editions but these were dropped in favour of Ultimate/Premium. There has always been an Ultimate/Pro/Premium edition of Zorin. And for clarity, whenever Ubuntu had an update to their LTS releases, such as 18.04.1, then Zorin would have released 15.1, and so on, but not necessarily based on the same updates that Ubuntu released. So Zorin started off as 12, but with updates became 12.4. Zorin 15 had 3 updates, so the last version of 15 was 15.3.