Because time doesn't work the way you think it does.
It passes at different rates for different people depending on things like how fast they're going compared to you or how strong gravity is.
In the example Katie spends 50 years travelling by Shannons' watch but for her only half a year has passed.
A good example of this from real life are muons.
These particles are created high in the upper atmosphere by cosmic rays but they're very short lived so even at speeds close to the speed of life we should hardly see any reaching us on Earth.
But because they travel so fast time is slower fro them and many many more reach the ground before they decay tens of thousands more.
see here:
http://hyperphysics.phy-astr.gsu.edu/hbase/rel ativ/muon.html