That's because light outside the particle horizon has started travelling while it was still inside, but has yet to arrive. The particel horizon has retracted beyond the starting point since then, but the light was already underway and is still inside the particle horizon. This also means that the observable universe will diminish to become as big as the particle horizon once all the light that is currently travelling has arrived, but by then the particle horizon will have retracted even more and the "new" observable universe will still be bigger than the "new" particle horizon.
Example for the sake of clarity: some particles were created, 1 lightyear nerer to us than the particle horizon particle horizon, which was, let's say, 1000 light years away. Over the next 499 years the particle horizon has shrunk by 400 light years, now the emitted particles are still 500 light years away but the particle horizon is 400 light years nearer to us than the place the particles came from is. In 500 years those particles will hit us and we will see a point that is effectively beyond the current particle horizon (which will have shrunk even more by then)