Understanding Google Places & Local Search – Developing Knowledge about Local Search

Here are two screen shots of on the Onebox returned for “city + st” search on Google today. Note the differences in map size and the inclusion of the “get directions” field and button in one of the results. I have explored a number of hypothesis as to why & when the difference occurs (existence of adwords, proximity to searcher, population) but none of them consistently offer an explanation. Ideas?