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  I'm not sure if you're asking how it knows, or what it's trying to do, having read your intentions to get up. 
 Cat's and dogs (pets) are very perceptive with regard to behavior and smell.  It's just a possibility that, as a leading member of the pack or pride, the cat senses your mental agitation, or hears bodily agitation, or smells your tension or adrenaline increase as you think about all the things you need to do, which may seem like you're nervous or agitated and in need of 'chilling out', so the cat responds by doing what cats do instinctively, to either find out what the problem is or to calm you down, or maybe to find out if it involves that animal.

A dog might get excited just before you get up for the same reason, in that it knows the signals you give off before you get up. In a dog's case it gets excited because it wants you to play. Cats in the wild like to be very calm because cats hunt by ambush, which requires mental and physical calm to be as stealthy as possible. It's a sign of safety too. So if you're agitated, that alone might be cause for trying to calm you, or find out the problem. If the pride is moving, then it jumps on the bed for more information. These all follow the premiss that the cat is good at anticipating your behavior, which really isn't any stretch. We anticipate the behaviors of others. We know animals do. Their senses can by thousands of times more perceptive. This is fact. This is how the cat knows you're about to get up. But having determined that the cat is interested in anticipating your behavior of getting up, the question remains what it hopes to gain by jumping on the bed.with you, but we can be certain that it wants more information. Cats are nothing if not curious. The multitude of cat videos prove that cat behaviors can be very difficult to put logic or reason to. Guessing what a cat thinks can elude you for its entire life, but they do develop habits, just like people. Animals that know you are even more amazingly perceptive. They really can predict an onset of seizures, cancer, emotional agitation and other personal details that are kind of embarrassing. If you want to get into what it wants by predicting this particular behavior, try to come up with different reasons for getting up. Try altering your thoughts or maybe pay close attention to the sounds you make. Try not getting up and then pay close attention to its behavior without touching it and try to keep your mind blank. See what it's trying to find out, or if it just wants the warm spot you leave behind. Cats like high places. Try giving it a higher comfortable place to sleep and to watch you and maybe it has no need to.jump on the bed. You've got to figure this out on your own but you've got plenty of ideas. :-) (I just deleted pages of boring commentary and thought, experience etc. You've welcome.) Animal behavior is very interesting isn't it?