Accuracy is an integer, the higher the better :
1 : estimated as average from numerically neighboring postal codes
3 : same postal code, other name
4 : place name from geonames db
6 : postal code area centroid
It seems accuracy describes where the values have been taken to calculate the distance rather than how much accurate the distance is.
Can someone please give more insights into what does it represent and which values means the more accurate distance while using query_postal_code