Its a combination of the speed of light and how inflation has varied the size of the universe. Light’s only been able to travel about 14 billion light years since the universe began but those further regions used to be closer so light from them was already part of the way here when they vanished over the cosmic horizon.
Wait, now that I think about it, the observable universe have precisely that length because the speed of light, doesn’t it?
Its a combination of the speed of light and how inflation has varied the size of the universe. Light’s only been able to travel about 14 billion light years since the universe began but those further regions used to be closer so light from them was already part of the way here when they vanished over the cosmic horizon.