I'm unable to find the source of this quote, has he ever said it?
As his only son, and colleague with him at Netscape from 95-97, I can attest that my dad did indeed throw that quote around, on more than one occasion. I'm fairly confident that he originated it (he was fond of coming up with clever quippets), though I haven't been able to figure out how it disseminated so widely over the past couple of decades. I'll keep looking around in old web archives and mails to see if I can dig something up.
Reference from David Karlton's personal blog at karlton.org: https://www.karlton.org/2017/12/naming-things-hard/
Tim Bray says on Twitter:
Phil [Karlton] said it, I reported it, someone else added on the “off-by-one”.
Leon Bambrick replies that he added off-by-one.