Weird Comments

I get quite a few anonymous comments to random posts. They are random in that there is no apparent correlation between the content of my posts and their comments.
Examples:
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Anonymous has left a new comment on your post "Carnival of Evolution statistics":

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Clearly the point is to promote some site (yes, I have changed the links). But why these robotish comment with no relevance to my blog posts?
None of these comments ever get through, because I moderate submitted comments to posts over a week old. And when they do get through to newer posts, they always delete them themselves right away. Why?
I really wish I knew.