This is one of our most common questions!
There is a funny technical distinction in the crossbar world. The listed weight rating is called the dynamic weight rating. This is the weight the crossbars can support while the car is in motion. So, the listed weight only needs to be enough to support the weight of the tent itself, since you won't be riding in the tent while heading to the campsite (right?).
Once the car is parked, that dynamic weight rating becomes a static weight rating. The static weight rating is roughly 5-7x the dynamic weight rating of the crossbars (while you are sleeping in the tent). This gives you plenty of excess capacity to start piling in the tent for a good night's sleep once you hit the campsite.