I second what Peter said. As they said in the Pirates movies: "It's more of a guideline than a rule". Not all vendors offer all of the IEC languages, and when they do they are usually customized/extended. These changes aren't necessarily bad; it often means that they've added additional functionality. Sometimes, though, the differences are just "we did it this way before the standard, why would we change?".
To claim to be compatible with the standard, I think the vendors have to include an IEC 61131-3 compatibility statement, which lists all of the ways they differ from the "official" way of doing things. You could theoretically use this to help determine how close a given platform is to another, but I'm not sure how effective that would actually be.