Probably a common slang error. From looking at the document's history, I suspect there has not been an "ANSI B18.2.l2" published in decades. In IHS the title is "ASME B18.2.12" as far back as 1972. Same as with B16.5 - it hasn't been an ANSI document for decades but some people talk about "ANSI B16.5" flanges. Simply put, it is an ASME document controlled by the relevant ASME committee and published by ASME. With approval and oversight by ANSI as far as the document's modification process goes (but not technical content), true, but owned and published by ASME.

no substantial difference