'Straight Outta Compton' -- Casting Agency Admits Mistake ... Not All Poor People Are Fat and Dark

The people behind the "Straight Outta Compton" movie say they're NOT just casting overweight, dark-skinned black girls to play "poor" people in the movie ... telling TMZ the casting ad was an "innocent mistake."

The ad went viral -- with the casting company inviting L.A.-based women to apply for one of several categories of extras that will be featured in the film.

The casting agency used an A, B, C, D scale to group the women. Here's the breakdown:

A) "Hottest of the hottest. Models" (open to women of any race)
B) "Fine ... Beyonce is a protoype here" (light-skinned women)
C) African American girls ... Medium to light skinned with a weave
D) "These are African American girls. Poor, not in good shape. Medium to dark skin tone."

People were upset ... since the "D" group was the only group associated with an economic class -- "poor" -- and it was made up of only dark-skinned, out of shape black women.

But we spoke with a rep for Sande Alessi Casting who tells us ... the ad was an "innocent mistake" -- and when it comes to casting the "poor" people, they're also looking for women of various skin tones and body types.

As for people who claim the A,B,C,D grouping system was racist -- Sande Alessi Casting says it's the usual method they use to look for different types of people for any project and it wasn't meant to offend anyone.