Complex sentence: The election of "gungnyeo" usually occurred one in 10 years, but there were exceptions; the way of electing "gungnyeo" and their social status differed from time, so the process was not systematized.
Split into simpler sentences: The election of "gungnyeo" fundamentally occurred one in 10 years, but there were exceptions. The way of electing "gungnyeo" and their social status differed from time, so those were not systematized.