I too don't think people mean to be rude, but I also tend to take offence to these kind of comments.

When I told family I was pregnant (with the baby we miscarried before this pregnancy), I got the comment 'yeah I thought you were' from a few people, funny thing is I was only about 4 weeks pregnant, how could they possibly know?! I was 7 DPO on our wedding day and feeling completely well, we found out we were pregnant 3 days later.

I think it is just the first random comment that comes to mind most of the time.