Marketing and self appointed Foodies who write for glossy food magazines....they tend to create a "fake" buzz well before a place actually boils their first pot of water.
Here are two concrete examples: Jimmy's BBQ was labelled "Best BBQ Restaurant on the Main Line" by MainLine Times way back in 2007. Jimmy's just opened a couple months back and as you will see elsewhere in CH it has horrible reviews. Prior to opening it was just a small catering outfit. Jimmy's owner must have either paid MLT staff or is intimately connected to them.
Maia was also rated "best" by Gourmet almost a whole year before it opened its doors.
Perhaps these days, they judge how good a restaurant is by looking at the size of the contractor's check?