The baggage allowance is indeed just a maximum allowance, and charging for amounts beyond this is a combination of a commercial consideration, and a mechanism to ensure that the average baggage weights per passenger used in calculations are pretty much adhered to. Anyone who has travelled in Asia, where maximum allowances are treated very casually, will have seen some with huge amounts; western carriers often have different and enlarged allowances on Asian flights, for competitive reasons (the Gulf carriers developed their market considerably by allowing substantial baggage weights).
Those of us who have departed Los Angeles for London or New York may well have seen the Hollywood Divas at the First Class counter checking in about 10 cases, maybe including three hatboxes, and the revenue manager looking very pleased .....
The averaging technique used to assess baggage weight is well used throughout aviation, assessing the average number of no-shows so you can overbook and still be OK is another. Different carriers have different approaches, and the best practice ones will do periodic spot audits with a weighing machine to ensure they are appropriate. Averaging of carry-on weight is done the same way.
Baggage weight is just a small proportion of passenger weight + baggage weight, and passengers weigh very variable amounts themselves of course; this is subject to another averaging technique. Some use different average weights for men, women and children, as that split is readily available from the reservations. Other carriers use different average weights between summer and winter, when passengers come on with different quantities of outdoor clothing.