These guys ride with different tyre pressure every race. Body weight only plays a small part in that (because that should be relatively constant throughout the season). There are not many riders that get their tyre pressure right from the start, and often they will shout to their mechanics, to lower or raise the pressure of the tyres next time they change bikes during the actual race.
It's all about the course (corners, obstacles, climbs, straights), the surface (mud, sand, cobbles, risk of punctures), the weather (slippery, hard / soft dirt road), the type of tyre (in combination with all the other factors) and skill (where the rider feels most comfortably on the track in order to make his move or even where he feels least comfortable and is losing time), that all decides what tyre pressure they prefer. A rider that is proficient in riding long passages through the sand, will lower the pressure, to get more grip in the sand and not "sink" into the sand that much so he will be able to ride through the entire sand passage. A rider who is not good at riding through the sand, and needs to get off and run through the sandbox almost every time, will do good to RAISE the pressure in his tyres, so he can make up for the time he lost in the sandbox (where he has to run) on the harder parts of the track where he can get more top speed and burn less energy on higher tyre pressure (with the risk of crashing in a corner due to less grip).
It could be very possible that a rider like van der Poel rides with more pressure in his tyres, since he is technically superior to the entire CX peloton (save Meeusen perhaps), and can afford to take the same obstacles with more pressure, which in turn helps him save energy and get a higher top speed on other parts of the track. This is just a theoretical musing of mine, i'm not saying this is actually the case, but something like that could potentially be the case. Or maybe it's the other way around and maybe the more technically skilled rider decides to put even less pressure to be able to take time on certain obstacles even more (such as a sandbox or sharp corners) and ride away further in those sections.
For any certain race, a rider might prefer a different type of tyre, which in turn gives him better grip on certain parts of the course, which in turn allows him to put more pressure in the tyre compared to his rivals, which in turn raises the risk of getting a puncture, etc etc. Or the complete opposite, depending on weather, skill, and numerous other factors.