Yeah definitely a marketing thing. About once a fortnight you get a Tripadvisor newsletter with "The Top 10..." whatever.
According to Tripadvisor reviewers:
Henry Doorly Zoo
Ranked #1 of 66 attractions in Omaha
St. Louis Zoo
Ranked #4 of 117 attractions in Saint Louis
Cape May County Park & Zoo
Ranked #1 of 5 attractions in Cape May Court House
Riverbanks Zoo and Botanical Garden
Ranked #1 of 40 attractions in Columbia
Memphis Zoo
Ranked #3 of 85 attractions in Memphis
San Diego Zoo
Ranked #6 of 193 attractions in San Diego
Sedgwick County Zoo
Ranked #1 of 21 attractions in Wichita
Brevard Zoo
Ranked #1 of 14 attractions in Melbourne {Florida, of course

}
Cheyenne Mountain Zoo
Ranked #3 of 72 attractions in Colorado Springs
Audubon Zoo
Ranked #12 of 152 attractions in New Orleans
Yeah, I'm going to go with "a list cobbled together by an intern" too - and one who couldn't even get a good geographical spread of zoos in America to keep their reviewers/readers happy.