It's a fair question.
There are two factors which determine the number of ad pages to editorial pages. The first, and most important, is a formula that we use to determine the proper ratio of editorial content to ads for a particular size magazine. The scale starts with a few more editorial pages than ad pages at our smallest magazine size, which we haven't used in ages. As we get more ads, we add more pages to the magazine.
We try to keep the ratio around 50/50 until we get to magazine sizes above 120 pages, at which point the second factor begins to come into play: How many pages can the staff generate in the time before we have to ship the magazine. As a general rule, we have more content than we have pages to fill, but last winter we weren't really prepared when we hit 140+ pages. Luckily, this year, we have some contingency plans in place.
The 1and1 ad that ran in this month's issue doesn't fit into that formula though. It's more of a bound-in insert than an actual ad (you can tell because the page numbers skip the entire ad section). I didn't even know there was an ad there until we got our issues a few weeks ago.
If you have any other questions, feel free to ask!