It's just the naming convention for FF.

Lightning = Thunder
Fire = Fire
Ice = Blizzard
Wind = Aero
Water = Water

The other 3 elements, Dark, Light, and Earth come in a lot more flavors throughout the series, so it's hard to give them a solid naming convention. The best Light spell is usually Holy, and Earth usually only comes from summons, or the Quake spell which I think is usually more of a Blue Magic spell in later games, earlier on I believe BLM got the stronger elemental spells like Tornado and Quake though.

Personally, I hope they remove the I, II, and III BS and actually start naming them properly. Thunder, Thundara, Thundaga, and so on and so forth.