The meat keeps cooking after it leaves the oven, and the juices are not going where you think.
Everyone is told to rest meat and almost nobody is told why, so the instruction gets treated as a superstition and quietly skipped. It is not a superstition, but the usual explanation for it — that the juices "redistribute" back through the meat — is not quite what happens either.
The outside of a roast chicken coming out of the oven is far hotter than the middle. Heat does not stop moving because you have opened a door: it continues to travel inward, and the internal temperature at the thickest part carries on rising for several minutes after the bird leaves the heat.
For a 1.6 kg chicken that rise is about four degrees. For a large turkey it can be ten or more. This is why the target temperature you pull at is lower than the temperature you want to eat at — pull a bird at 75 °C in the thigh and it will finish somewhere around 79 °C, which is dry.
It also means the resting period is not idle time. It is the last part of the cooking, and it is happening whether you plan for it or not.
Muscle fibres contract when heated and squeeze water out of the cells into the spaces between them. Cut into meat at that moment and much of that loose water runs straight out onto the board — this is the puddle that appears when an impatient roast is carved.
As the meat cools slightly, the fibres relax and reabsorb some of that water, and what remains becomes viscous enough to stay put. The juices are not travelling anywhere in particular; they are being taken back up and thickened. The practical effect is the same as the folk explanation, which is presumably why the folk explanation survives.
This is the part that trips people. Wrapping a bird in foil to keep it warm traps steam against the skin you spent ninety minutes crisping, and it will be soft by the time you carve.
Instead: lift the bird off the tin so it is not sitting in its own liquid, put it on a warm plate or board, and tent it loosely with foil that does not touch the surface — or leave it entirely uncovered in a warm kitchen. A 1.6 kg chicken loses remarkably little heat in twenty minutes. It is far more likely to be too hot to carve comfortably than too cold to eat.
Use the twenty minutes rather than waiting them out. It is exactly enough time to make gravy from the tin, drain the potatoes, and get everything else to the table so that nothing has to wait once the bird is cut.