It's not only a matter of "big numbers", but also of confirmation bias.

We're never paying any attention to irrelevant ads - we're ad blind to them. But the moment we see a very relevant ad to a recent conversation, that will inevitably pop up.

"See, this one ad is tied to this conversation I had yesterday: they must be listening", while in reality it's more like "See, this one ad out of 100s I've seen in the last day is tied to this one conversation out of 10s I had in the last day".