My guess is, those people instinctively want to avoid the dilemma of how to pronounce the initial vowel in "Adler" (at least 4 possibilities), so they fall back on Alder, which has a "consensus pronounciation".
My own last name, Sieber, (correct pronounciation like "Sea'ber"), is generally copied as "Seiber" and pronounced "Cyber" in Anglo-Saxon contexts, whereas Germans can't resist adding a final 't' to it!.