Aparentemente você não é a única pessoa que se perguntou isso; a FAA realmente tem em uma FAQ :
What is the significance of a runway 8069 feet in length and why are two different aerodrome symbols used to depict hard surface runways on Sectional charts?
For purposes of airport depiction, specialists represent a runway between 7970 and 8069 feet in length as 8000 feet, which equates to a line 0.192 inches in length on the Sectional chart scale. In this case, a circular aerodrome symbol is used.
If a runway is between 8070 and 8169 feet in length, specialists round to 8100 feet, which equates to a line 0.1944 inches in length on the sectional chart scale. This line is too long to fit into the largest circular aerodrome symbol FAA has available. Therefore, specialists place a line-work around the runway pattern forming a polygon (enclosed shape) for anything over 8069 feet in length.
Specialists also place these polygons around the runway pattern of aerodromes with multiple runways that are less than 8069 feet, in cases where the multiple runway pattern does not fit into the largest, circular aerodrome symbol.