I've been told, although without sources to back it up, that the original load specs for the 9x19mm Parabellum were somewhat hotter than the metallurgy of the time could handle, so they were dialed back a bit; unfortunately, this loss of power also made the Luger's mechanism somewhat less than reliable. Since the 9x19 was originally designed for the Luger, I find this debatable, but plausible.