Not all numbers are used. Some are left out to make room for statutes that are passed later. Sometimes numbering jumps because the area of law changes. Sometimes statutes are repealed.
If you are looking for a specific code and it doesn't exist, the statute may have been renumbered or perhaps you are incorrect about the number of the statute you need.
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There never was a PC 12400. Maybe you are thinking of PC 25400, formerly PC 12025, possession of a concealed gun. I admit that the numbering of the Penal Code in particular is totally confusing. If you can't find a number try a word search if you are on line or if in a library ask for help.
In my experience, the online listing you are talking about is accurate. The leginfo site does not have certain codes, for example rules of court and local ordinances. But it does give you access to the codified statutes. Often, you will see a gap in numbering. That does not mean that a section is missing, it just means that there is no longer any code of that number or that there never was.