The audition is just the casting director and the camera. At the callback, you add the director and the producer and the product manager, people from the agency, from the client, there may be ten people, and it's casting by committee. At the callback, I read with three different groups of women, and I always played The Dumb One, except once, when they had me read The Doubter. I stumbled over 'snack stop'. They cast three extras and three principals, and four of them were women I'd read with.
Commercials represent a huge investment.
It's the visual representation of the product, and mistakes are horribly expensive. No one wants to be the one to blame, so they make the least offensive choice. You be all you can be every time, and pretend you don't know any of this. Remember, if a national commercial runs, it can change your financial picture. A friend of mine did a Crisco commercial.