The discrepancy you’re encountering in word count may be due to the fact that picture books serve a wide-ranged audience, from toddlers to tweens. For the record, there are no specific “rules” regarding word counts, nor should there be. But perhaps this breakdown will help you navigate these cloudy waters:
Young picture book: For ages 2 to 5. Short, simple texts/concepts and young, bold illustrations. Example: Guess How Much I Love You You, approx. 400 words.
Standard picture book: For ages 3 to 7. Generally illustrated stories. Example: How I Became a Pirate, approx. 1000 words.
Older picture book: For ages 6 to 9. Somewhat more sophisticated subject matter and approach to illustrations. Example: Harvesting Hope: The Story of Cesar Chavez, approx. 2500 words.
Illustrated story collection, Illustrated historical fiction, or Nonfiction photo-essay: For ages 7 to 10, although the ages vary for this category; “All ages” may apply. Stronger emphasis on text than illustration, and the format is typically of a larger picture book size. Example: Lives of Extraordinary Women, approx. 16,500 words.