Correct APIVersion but addon "out-of-date"?

[Noobanidus]

So, someone recently commented on one of my addons (uploaded within the past few days, and brand new) to inform me that it was marked as "out-of-date" in-game. I assumed either it was an error on my part (typo or forgot to modify the file when I copied it from my template) or an error on their part.

However, the manifest appears correct http://i.imgur.com/haAZ6PC.png

But the addon is marked out-of-date http://i.imgur.com/RRq8Rnf.png

However, another addon which I wrote today has the exact same manifest http://i.imgur.com/lvkHVX0.png

And is not marked as out-of-date http://i.imgur.com/Ws13wmK.png

As an experiment, I copied the manifest file of the functional addon over the top of the existing one, changing the description and title, but it still is marked as out-of-date.

I can't see what I could possibly be doing wrong here. I'm obviously missing something, and hopefully it's something simple, but I just can't put my finger on what it is.