This is just a guess, but could they be using some algorithm based on analytics data (say, which cities are zoomed in on and searched for the most and/or A/B testing) to determine which cities should show up on zoomed-out maps? They can't show every town and city at once without things getting very cluttered, and it seem very Google-like to relentlessly optimize based on user data.
Or else they're just going off some combination of population size and avoiding clutter/overlap.
I opened up google maps to stare at it for a while, and one thing that jumped out at me is that I think they might be using different algorithms for different countries. Using what comes up as the default zoom for me (four notches from the bottom, enough to make the USA about 600 pixels wide) I see that the US has cities marked relatively sparsely, whereas they're much denser in Mexico.
Scrolling down to Australia it looks like the algorithm is far from perfect; it shows all the state and territory capitals except Adelaide, which is an odd omission.
Recently some cities and certain addresses have been actually moving, as well. There were a couple of months where it decided my house was actually about 5 miles from where it is, which made giving directions more complicated. (I had to specifically tell workmen and visitors to make sure they used Yahoo Maps or some other map source to avoid misdirections).
More amusing, for the last couple of weeks Google Maps has placed Randolph Vt right in the middle of Lake Champlain.
I can see that some of the cities are popping in and out - in the top example, Highland park by the lake in the upper right, and Lasalle at the intersection of 39 and 80, but it's really hard to see them with the labels moving around. Might have been easier for him to make his point if he could add in some red circles around the affected towns.
Actual, maybe the cities are being hidden due to label placement? That is, there's not enough room to display all the cities with their labels, so rather than have them overlap, the algorithm chooses to hide the city if there's no room to place it.
I bet if you zoom in on a missing city it'll show up.
never put much thought into it, but I've assumed that they changed based on whether I was logged into my google account at the time, since I noticed that the non-notable town i'm from usually shows up when zoomed out pretty far, to put it amongst labels of much more populated places.
when I look at my home address (rather rural) some local place names (way too small to be called a city) do indeed appear and vanish. I've attributed it to some kind of rotation algorithm. Some of these place names are so obscure, I've barely heard of them.
Seriously? I suppose if one has a lot of time to waste, he can obsess about Google's label-placement algorithm, and how it seems to change from time-to-time. If one is really obsessed, he can even take screenshots every day, and put up a web page about it, expecting others to be equally obsessed. Or, one might consider getting out once in a while.
