Hey cointalk, This Fair Kennedy looks like a mighty Fair deal http://www.ebay.com/itm/1964-US-Ken...d=100009&prg=1088&rk=1&rkt=5&sd=321174390395&
Because, the Lower grade coins from the Modern era are very uncommon. not many Kennedys, for example, were used in daily commerce so much, so they stay within the XF-MS range. However, it's that the Kennedy here was more worn than other coins, and thus 'rarer.' It gets more valuable as the wear gets more extreme. There are competitors in PCGS Lowball sets, where PO-1 counts the as MS-70, and MS-70 counts as Po-1.