The current rules were introduced in 1995 as part of EU Directive 95/48/EC. The EU standard is based on the respective component of the DIN 70020 standard which was published even earlier. These both mandate a 90% full fuel tank and all lubricants, oils, spare wheels, tools etc. The EU standard effectively added the 7 Kg luggage and 68 Kg driver allowance. The idea behind the measurement is to provide a mass close to the actual vehicle in road trim. The 90% full fuel tank works quite well in this regard. Using a fixed Kg penalty would be disproportionate as DRZ explained.
Currently the manufacturer’s offer a smorgasbord of minimum and/or maximum equipment specification benchmark weights, ranges and other figures. There is a good reason for this, as it avoids them having to measure each possible equipment combination. For large vehicle ranges that are sold throughout the world, there could be thousands of weight combinations for vehicles even before you consider optional equipment, as each NSO/importer can ‘negotiate’ their standard equipment packages with the factory to best suit their market. With some effort it would not be impossible for them to provide indicative weights of each optional equipment item, so you could add them together yourself and calculate a new estimated kerb weight.
Something to bear in mind is that EU Directive 95/48/EC allows for a -/+ 5% production tolerance. A car that the manufacturer offers as 1700 Kg EU could weigh anywhere from ~1620 Kg to ~1780 Kg.
