A V-20 hull measures 20'-6". So I think sometimes a lot of sellers round up and call it a 21' while some round down to 20' when they describe them in their listings. There is no difference in length between any boats that are V-20's throughout the years.
As Destroyer said though these are not to be confused with a V-21 which like he said was made towards the end of the run. The hulls from what I am told (never seen one in person) are identical to the 20'6" long V-20 but they just added a motor mounting bracket into the transom and called it the V-21
