Okay...I've got a decent grasp of XHTML...therefore would it be safe to say I also understand XML therefore be able to list this as a skill on my resume. Sure it'd be a beginner understanding but XML and XHTML are basically the same thing. I've never used XML but have went through alot of sites and stuff and have a basic understanding of it and how it works.
If you know XHTML then pure XML is practically a no-brainer from that point. You might need to spend some time familliarizinng yourself with different schemas (RSS, etc.) and other things like XSLT, but for just pure, basic, straightforward XML, I'd say you'd manage just fine.
I think it's pretty safe to include it on your resume. If somebody does call you on it, just say that you have "some" experience with XML, and more extensive experience with XHTML. If you can describe what you've done with XML then I don't think it's a big deal to list it.
Now, just imagine a job where they're looking for a Java programmer. The HR office gets 2 resumes. One says:
Java - Expert
HTML - Intermediate
C# - basic
XML - basic
the other says
Programming Tools: Java, HTML, XML, C#
Now, the company knows they need a Java programmer, so they're both ok there, but then the rep thinks, Hmmm, what if we decide to do XML programming in the future? He would then say, let's interview the second guy, because he knows all those languages, while the first guy only has basic skills in XML. Now, it may turn out that once the guy gets in for the interview, he has to say I only have basic XML skills, but by then he's already impressed the interviewer with all his other abilities. Advantage, him over the first applicant.
When people do that on Monster, it's because they're really focused on a job that only uses the skills they call expert - and vice versa, the employers that search monster are looking only for certain people with expert skills in specific areas.
Further, Monster carries a huge database of resume, I believe its around 33 Million resumes. If they do not breakup the skills into different levels, the search results would be too overwhelming for employers and less targeted.