[LeifCarrotson]

Bad for you != bad for the advertiser, and bad for the advertiser is not necessarily correlated to annoying to you.

While your particular case may need just one coffee machine, there are other scenarios. If you just bought a coffee machine, perhaps it will be defective and you'll need a replacement. Perhaps you didn't know about the one they're showing you and will return the first one to buy this one. Perhaps you are just getting into this coffee stuff and realize that a second one for the office would be nice. Perhaps you're buying a few to compare them.

It could be rational to show you the ad instead of a random individual if the sum of these scenarios is greater than the rate at which random individuals buy coffee machines.

Or, yeah, it could be that their ad-showing algorithm just tells them you have the word "coffee" in your recent browsing history.