The reason is because sRGB (IEC 61966-2-1:1999) defines the transformation from CIE XYZ to sRGB using a matrix with 4 decimals where ITU-R BT.709 only specifies the primaries and whitepoint which allows derivation of transformation matrices at machine, e.g., double precision.
The reason is because sRGB (IEC 61966-2-1:1999) defines the transformation from CIE XYZ to sRGB using a matrix with 4 decimals where ITU-R BT.709 only specifies the primaries and whitepoint which allows derivation of transformation matrices at machine, e.g., double precision.