Do you approve all comments left on your blog? I know that Akismet plugin normally does a good job of blocking spam comments to your blog, but what if it lets something through that appears to be legitimate, but not related?
The person making the comments does not have a related site but the comment is good, but general?
Eg Comment from my tech blog: Good but not related.
"Heya i'm for the primary time here. I found this board and I to find It really helpful & it helped me out a lot. I hope to give something back and aid others such as you aided me."