One of the design aspects of loading systems (ie how to figure weight and balance for a flight) is to embody tolerance checks so that minor errors in practice (in this case, a "wrong" baggage value) are covered by the tolerance flexibility of the loading system. There is a penalty in that the system, itself, will now have inbuilt errors but we can balance one against the other to end up with a useful, load error tolerant system. End result is that a few bags left behind will not create a significant hazard.
If we didn't go this way, it might be necessary to redo the sums for every change prior to flight. Not too much of a problem for a C172 but can be a real nuisance on an airline B747 operation.