Not sure about the NG, but on the 'classic the gauge tolerance on all tanks is 2.5% of full-scale reading, i.e. 0.025 * 7000 = 175kg. The CTR tank is bigger than the wings tanks so the possible error is larger, thus potentially more significant.
When trying to measure small amounts in the CTR, the existing error can be at least as big as the amount you're tyring to read, so accurate readings are difficult. When we uplift anything into the centre tank, company SOPs are to uplift the reqd volume of fuel from the bowser and check the gauges are within limits, rather than using the gauges to uplift only to find when airborne that the CTR tank's error is in the 'wrong' direction; thus leaving you close to 200kg short.
Sorry if this is barking up a slightly different tree, and that it doesn't really answer why there is an error in the first place, except to say perhpas that nothing's perfect.