[classicman]

There is also another reason. One problem is that sometimes the ads are provided in what is called a full bleed design. This is when the ad/image appears to run off the page on all 4 sides. There are also partial bleeds and no bleeds. (Hope they are self explanatory.)

Many times the advertiser or more typically, their agency provides the ad to the magazine and there is no opportunity for the magazine to alter it by putting a page number on it. If the ad is a partial or no bleed, its much easier.