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[edited by: anallawalla at 2:32 am (utc) on Sep. 23, 2009]
[edit reason] delinked search example [/edit]
I delinked your example search (we generally don't allow specific searches but this one is educational).
I can only guess why the ZIP code is generating the map. I'd expect both examples to *not* show a map, as the search is too ambiguous to convey whether it relates to a sale of a good or service. e.g. apartments for sale, apartments to let, etc. The search might well be for historical research or someone looking to share an apartment. Being a borderline query, I am guessing that the ZIP code tips it into the commercial intent area.
OTOH, an obvious service or business such as any of the trades nearly always produces a map for a large locality.
In one local city search I tried, there were three results...
1) BusinessName Apartments, with its own website...
2) FancyName Apartments, apparently a now-vanished listing on a nearby city rentals listing page, linked in the serp...
...and... particularly interesting...
3) one was a classified ad with the title Cityname Apartment, with the only contact info being a hotmail address. I'm thinking the ad was perhaps a Google Base result, as there's no url of any kind associated with it.
As to why only the query with the ZIP returns the map, apparently Google is looking to provide localized rental classifieds, and ZIP granularity is probably what they consider useful.
They're stretching hard to fill even three slots, but it looks like they are trying this out. I don't think Craig's list needs to worry just yet. ;)