What happens when you roast a piece of meat, is that the hot temperature causes the proteins to coagulate, contract, stiffen, etc. This acts to squeeze the juices towards the center of the meat, as the hot surfaces are all around. This is where the misconception that searing meat "seals" in the juices comes from.
What happens when you pull meat out of the oven, is that the temperatures even out. The "hot" temperatures on the outer surfaces still move inwards, as well as radiate outwards away from the surface. As this happens, the juices in the meat redistribute throughout the meat, instead of being forced into the center by the coagulation of the protein.
The larger the cut of meat, the more noticable this is. Remember that the tip of your thermometer needs to be in the center of the meat, but not touching bone (if present).
Where I work, when we roast pork loins, we pull them out when they register 150°F, stick a probe thermometer in it, and let it sit for about 15 minutes. You can actually watch the temperature continue to rise as it rests. Once the temp holds steady, then we usually slice it for buffet service.
You are exactly right, that you need experience doing this, not only knowing when to pull the roast out of the oven, but, the intricacies of your oven, any hot spots, does it cook at the indicated temperature, is it convection of conventional, etc. This is why the pro's say "Cooking is an art, Baking is a Science".