The Pokemon Company likes to make certain cards for reasons. The LEGEND cards represent elements, duets, not Types (necessarily); the Lugia and Ho-oh ones were mimicking the Entei and Suicune, as well as Fire-Water being way better than Normal-Water (TCG wise, and Psychic has a really common weakness); (the only Normal LEGEND card is Raquaza).
For Others Think about this: they all came out in the D/P/Pt/HG/SS era, right after the RSE era, where dual types, dual weakness/resistances, and Delta Species Pokemon were introduced. Delta Species Pokemon are different types than normal (Ex: Steel type Pikachu), so it would make sense to have a Lugia be on a Water Type Card for these reasons:
- Lugia was a being of water, and they'd (most likely) never make a Delta Species Pokemon again.
- Having a Water Type Lugia card would also make for more Versatility for Lugia itself. Normal type was only super effective to the scarce dragon normal types, and Psychic Lugia was weak to Psychic, which was (and still is) way more common than Water Lugia's Lightning (Electric) weakness.
TL;DR, Game Freak Logic and to increase viability of Lugia. Notably, HG/SS era is the last time they did such things.
(I refer to game time periods as eras of said game by habit.)
Hope I Helped!