I believe the Skinny address checker takes congestion of the network into account, and I guess that less network capacity is allocated for use by Skinny customers than by Spark customers. For example I checked my bach in Otematata for Skinny 4G service over the holidays and it showed as not available, however it is available now. Spark 4G was available to order on both these occasions.
The population of Otematata can swell to 5000 people over December/January, but there are less than 100 permanent residents. If I order Skinny wireless now when demand is low (which I plan to) then I can be sure of having access to the service next summer when I want it.