You can actually go above the hook in weights but not much. If a wing calls for say: 180 to 225 hook in weight you can actually take it too around 250 if needed maybe even more.
The glider is not going to fall out of the sky. The worse thing that happens is it will have a slightly less glide ratio and you might have to come in a bit faster to flare but those numbers won't be that noticeable on a wing say like a Falcon 190 or larger.
Now if your talking about a smaller wing like a 150 or 170 then this might be a lot more noticeable if your still adding the same twenty five extra pounds or so because you have a lot less surface area and sail material regardless if the weight of the pilot is a lot less for a particular smaller wing then the 190.
A smaller pilot who weighs less flying a 170 compared to a pilot weighing more who flies a 190 can stretch his/her weight limits won't always be the same with the extra weight of say 25lbs for both wings because of the lack of sail area. It sounds like it might be based on pilot weight or hook in weight and the ratios are suppose to be configured for each wing but in fact your still adding extra weight (same weight 25lbs) to both wings but the smaller wing will handle a lot different then a larger wing carrying that extra 25lbs.
The reason for this is simply because the air itself stays the same. Now if a lighter pilot flies and the air would adjust itself to that wing then maybe it wouldn't matter but the air stays the same (density and temperatures) based on altitude or ASL and weather conditions so it's going to be the same for all pilots light and heavy thus acting differently on each wing and pilot flying.
The air don't care how much weight is tied to a glider it just knows how to react with the amount of sail cloth flying through it regardless and the less the material the faster a wing will have to travel to maintain it's optimum glide ration and handling capabilities given by it's manufacturer.
The weight specifications are given by manufacturers as a sort of best glide ratio and handling of the wing and it's stress factors and it's design keeping those numbers in a fairly large *zone* if you will, of minimums and maximums but they can go over these numbers or under depending on sail area of the wing and hook in weight minimums and limits.
That's why the wing I use to have was like a 747 and could probably carry a BBQ Grill, Keg of beer and the food to go along with it and still fly to BFE and back on a good day. I like larger wings simply for the fact I can stretch those numbers safely and still fly within range of good performance and safety of the wing and it's structure.