It's simple. The original arcade multisync used for MAME was the D9200. It remembered geometry - but it had a gap between 16.5 and 24khz. So the stock 15khz mode for, say, Pac-man is 352x288@50hz. You sacrifice refresh. If your monitor can do 15-24khz continuous, then you end up at 18-20khz. I forget the actual modelines those are.