There are three reasons for this:
When a company buys a page for an ad, they are literally buying that page. The magazine producer must put their ad on that space and nothing else. In the case of an ad that covers the lower outside corner, most magazines will leave the page number off instead of moving it over into their space, keeping the page format consistent. Multi-page "special advertising sections" will often include their own page numbers to make them look more like part of the magazine.
Page placement isn't fixed until just before the magazine is printed. Leaving page numbers out means the ad can be moved around without additional changes. Sometimes an advertiser may request to have the ad placed next to a related article. Since the article placement changes, the ad must move with it.
Sometimes the magazine cover is counted as page one, but labeling it isn't useful since you can probably find the cover yourself. This system is mostly seen on magazines where the cover is also the index, like Reader's Digest.
Posted 3320 day ago