I suppose I can slightly understand skipping Zorin 13 going from Zorin 12.
Yet, looking at the Zorin OS lineage, it followed a set numerical pattern until Zorin OS 15. Was skipping Zorin 14 as well just to make the reason for skipping 13 less obvious?
Following 15 is now back to the numerical pattern again. Zorin OS 16.
Will we skip 17 and move to Zorin OS 18?
Or should we bump the next version up to Zorin OS 20?
Personally, I think we should use Roman Numerals, instead:
Zorin OS XVII
“The reason is that this is actually the 15 version of Zorin OS. In between Zorin OS 1.0 and 2.0 way back in 2009, we released a version called “Limited Edition ’09” [so] we decided to take the opportunity to correct the version number with this release.”
That would account for Zorin OS 13... 
But what about 14?
Also from the above article:
Wait, what happened to Zorin 13 & 14?
As the previous stable release of Zorin OS was version 12 you might be wondering where versions 13 and 14 went.
Well, don’t panic: you didn’t miss them. Zorin’s lead developer explains why there’s a small ‘jump’ in version number for this release:
I think the version after Zorin OS 15 is Zorin OS 18 but it turns out that Zorin OS 16. I am curious the next version of Zorin OS 16 is Zorin 17 or Zorin 18 or Zorin 19 or Zorin 20. 
I am just upset that they left 14 out in the cold. I mean, fine... They made up for the Limited Edition 09 bumping all editions after up by One Version... that gets rid of Zorin OS 13. But 14 was a forgotten page in the annals of History.
They made up for one, so they should make up for 14, too. By making it.
I vote that the version following Zorin OS 16 should be Zorin OS 14. We're takin' it back.
Then, Zorin OS 17 should follow Zorin OS 14... Causing all to be well in the Cosmos, once again. It really is the only way to make it all make sense.
Maybe Zorin OS 12.4 should be named Zorin OS 14. 
For some reason that just made me think of this post by @Storm :
I second the motion... Then, we start a weekly group chat named "ZorinOCD"... Things have to be in order...lol
We could make a zorin os 14 based off of Arch linux just so we can say there is a zorin os 14
Funny how this thread came back from the dead. I have fully explained this in the dim and distant past, keep up people! The numbering format used to go up in 3's because they were LTS versions based on Ubuntu LTS versions starting with 6, then 9, then 12, then 15. Anything in between was a step release, not an LTS release. That is why the incremental 3's stopped at 15. Zorin 7 saw the introduction of the logo change from this:
to this:
It was with the release of Zorin 12 that Team Zorin decided to concentrate on only developing LTS releases, which is better for the ecosystem.
Ohhhh. That makes sense.
Just to add further clarity, note that Ubuntu's LTS releases come out on April in an even numbered year every 2 years with incremental releases every 6 months starting in the October of the LTS release year. Zorin 6 was based on Ubuntu 12.04, Zorin 9 on Ubuntu 14.04, Zorin 12 on Ubuntu 16.04, Zorin 15 on Ubuntu 18.04, Zorin 16 on Ubuntu 20.04, and currently Zorin 17 on Ubuntu 22.04.
That makes it all make sense. I reminded me of the Windows 9 situation at first but now that makes more sense.
