I think it was a marketing decision
by the people trying to sell Lebkuchen to non-Germans so they would at least have some idea what they were talking about if they didn't know what Lebkuchen was.
In reality, Lebkuchen is Lebkuchen. Gingerbread is "Ingwerbrot" in German and is what we know as gingerbread over here. We don't have a translated equivalent to Lebkuchen in the english speaking kitchen as we do for gingerbread.
The confusion is also wrapped around the sticky sweet doorknobs of that witch's house from the brothers Grimm. That house, the name which causes Marilyn to "Moe! Larry! Cheese!", is not "Gingerbread House" in German, but was translated that way in some versions of the fairy tales for English editions.
The various names in German are "Knusperhäuschen" (Little Crunchy/Crispy House), "Hexenhaus" (witch's house), and Lebkuchenhaus (Lebkuchen (literally, cake of life) House).
We call it Gingerbread House because the German equivalents aren't very warm and fuzzy to non-German speakers.
I had a discussion about this very topic with a Lebkuchen shopkeeper in Munich about 20 years ago. She was also perplexed why English speakers were referring to Lebkuchen as gingerbread. It was confusing to her since gingerbread was a completely different thing.
So to wind down the diatribe, while English speakers (and Germans talking to English speakers) will refer to German Lebkucken as "Gingerbread," it is not.