I am no physicist, so bear with ne here. It has to so with the expansion if space. One thing to note here is that space expands faster than light travels. How? Well, basically nothing IN the universe can move faster than light, but the universe itself can.

If space did not expand faster than light, then the size of the observable would indeed be equal to approx 27 billion ly end to end.

Imagine you are walking at a constant speed and whichever distance you cover is the observable universe. Now imagine that the ground you are walking on suddenly stretches forward several times faster than you are walking. You have covered the entire distance, but you have not moved any faster.

Or imagine a being in a jet. You yourself move at 0 km/h but your body covers the same distance as the jet.