The house tradition splits travel in two, and the split is by kind rather than by distance measured out. The third house takes short journeys, the ones a person makes inside a world they already know: to the neighbouring town, on an errand, among kindred and neighbours. The ninth takes long journeys, voyages and travel abroad, and it takes them in the same breath as religion, philosophy, learning, dreams and the church.
That second grouping is the more revealing of the two. The ninth is not the house of distance. It is the house of what lies beyond the native's own frame of reference, and a foreign country, a foreign god and a difficult book are all instances of one thing. Lilly's ninth carries long voyages and religion together for that reason, and a reading that separates them loses the logic that put them there.
The planetary significators follow from the signs those houses are natural to. Gemini is the third sign and Mercury rules it, so Mercury carries the errand, the message and the road. Sagittarius is the ninth sign and Jupiter rules it, so Jupiter carries the pilgrimage, the voyage and the foreign teacher. These are derivations from the order of the signs rather than assignments made in their own right, which is why the sources state them with less authority than the house significations they follow from.
Ptolemy gives foreign travel a chapter of its own in the fourth book, reading it from where the luminaries fall relative to the angles of the chart.
Sources Christian AstrologyWilliam Lilly with TetrabiblosClaudius Ptolemy