No worries! You got me curious, and having looked it up, I figured I might as well share what I had found. (My access to the OED is entirely internet-based, alas. One of these days, I'll live in a house big enough that I can afford to buy and then shelve the physical thing.)

"Ten-year anniversary" is definitely a less desirable phrase than "tenth anniversary," but I can see why the latter phrase (which makes sense) would morph into the former (which makes less sense) over time. "Six-month anniversary" is technically a nonsensical phrase . . . but I don't have any good suggestions for alternatives that would convey the same meaning. Sixth mensiversary? I should stop coining pseudo-Latin words and go to bed. :-)