Because Google can only find 2 advertisers that match the keywords it thinks your targeting for the region/time zone your in?
So? Google need to determine what your page is about and then match advertisers with it. Don't forget, what you see isn't necessarily what someone else sees (based on time and region). Giving a URL might help.
If your visitors are presented with 4 different adverts you might argue that it's more likely they will find one of them appealing than if they are presented with 2. Just because you see 2 ads, it doesn't mean that someone else isn't seeing 4.