According to https://proofreadmydocument.com.au/writing-tips/spelling-tips-the-doubling-up-rule/ and https://www.macquariedictionary.com.au/resources/view/resource/20/, we should spell the past participle of "develop" as "developed", and the present particple as "developing" (mult-syllable, unstressed final syllable of the stem). Goolge prefers it, too. Still, it provides us with over 23800 references for
Are they all wrong?
As for Bing,
have approximately the same counts.