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. 
I second the motion... Then, we start a weekly group chat named "ZorinOCD"... Things have to be in order...lol