Why do some Boston Celtics championship banners have the NBA logo in a full cirlce, but others do not?

Some years, 1981, 1968 for example have a full circle around the "NBA". Others -2008, 1986, have a crescent at the top %26amp; bottom. Kind of forming a basketball.|||they probaly just changed the style over the years|||The Celtics are gay like that....|||The logo might have changed between some of the championships, but the Celtics wanted to keep the same design the old ones had.