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i've seen similiar dynamics in conference program committees:
if you receive a lot of high quality submissions you just have to throw (often truly excellent) stuff away till you're down to a manageable size.
and that's not even a judgment of the individual submissions - often it's just a matter of making a balanced program.
According to a post by pg on HN accepts all applicants that they feel can make it through the program (ie the idea and team are ok). Therefore they are not going to exclude good applications because they have reached an arbitrary limit.