Let's Read B/X: The Moldvay/Cook Basic and Expert Sets for D&D

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Part 4: The AdventureFor some reason the maximum range of spells is also trippled. Or actually, for no reason at all. It just is.

I believe this is a holdover from the Chainmail / Wargames days, where the Fireball and Lightning spells (the first Wizard spells) derive from the catapult rules. If you think of spells as being things that were almost literally lobbed and had something of an arc to their travel, it makes sense that you have more range outdoors than underground.