...... they're either saying that the factory made an error on a number of airplanes, (incorrect scale calibration), or people are referencing the wrong document - sales brochure. If in doubt, reweigh the plane. An airplane flying at an uncertain weight, is most likely also flying at an uncertain C of G position, which could be every bit as serious a situation.
I hope that pilots are not using a sales brochure for flight planning - but nothing will surprise me. Sometimes the manufacturer will offer a fully equipped aeroplane with a suite of all possible equipment and the W&B report for that condition may well be included in the pre-printed POH format for that model with dated signatures. However, this isn't always the case and many aircraft will be provided with the absolutely minimum equipment possible and a W & B report for that condition. The manufacturer may offer all kinds of extras and for which they provide an STC free of charge. These could range from long range tanks, right seat toe brakes and to a purchasers customised choice of radio equipment. As Pilot DAR says these will require a re-weighing or a recalculation of basic weight/ C of G. and a new W & B report issued even for a new aeroplane. This is the report that should be used not the original. So, a worthwhile reminder by CASA.
I agree with Jan that a 100 Kg error for a LSA/Microlite/Ultralight is a massive error. Shocking if true.