Re: the ice cream analogy, Dungeon is not selling ice cream; there are not 31 flavors in every issue, there are 3 adventures, one of which is part of the Adventure Path, the other two which are left to represent facets of a large volume of settings and locations and times and places, as well as various levels of difficulty (ie appropriate for different party levels). So in effect, there are more than 31 flavors, and only room to offer three at any given time, one of which is going to remain the same for the next several months.
But each adventure isn't just supporting Forgotten Realms, Eberron or Greyhawk. Those aren't really flavors. The adventures are more aptly identified as either urban or dungeon-crawl, set in a port town or set in a war-torn country or set on the various planes, combat-heavy or RP-heavy, or any other more appropriate descriptor than simply "Forgotten Realms, Eberron or Greyhawk." It's an attempt to simpifly a complex issue to make a statement that "Dungeon 140 contains no Eberron or FR content." Can the content be made to fit with little or no modification in an Eberron campaign? If the answer is yes, then there you go.
If an adventure is set in a war-torn country, for example, does it add value to say it is set in Karrnath, or Amn, or Furyondy (three examples I've pulled at random from my passing familiarity to each campaign setting; as I said in an earlier post, I run a homebrew campaign, so if an adventure tells me it takes place in Impiltur and offers no other info it really means nothing to me)? Or does it add value to say it is set in a war-torn country, and then you provide the further details as you see fit for the campaign you run? Do the editors actually have to tell you the adventure takes place in Aundair for it to be useful for an Eberron campaign, or can you read the adventure and think to yourself, "I've been wanting my players to take a trip to Aundair. This adventure would fit perfectly!"
Please do not assume that my opinion is the official stance of the Dungeon editors; I have no affiliation with that magazine, except as a subscriber. But I must add that apparently the Dungeon editors are mostly happy with their current mix of adventures each month, as are the bulk of their readers, or else the editors would be madly scrambling to make changes post haste in order to deal with quickly slumping sales.