Resorts don't accept faxed requests, so guests who get their requests are generally the recipients of coincidence (or the fax was a follow-up to a spoken request at or after the time the reservation was made).
Since building 27 is slightly closer to the main building than 17 or even 16, and therefore 18 is much closer than either of those two, and since what impels Disney to upgrade a section or building to the higher-priced Preferred category is numerous (hundreds/thousands) of requests for a building/location - it would appear that enough guests over recent years requested those four specific buildings that the powers that be decided this would be a good idea.