For the first time in the competition's history, those who already had managers were eligible to audition earlier this year.
It means that acts who are professionally represented will appear in the upcoming series, as producers don't want to "miss out" on any talent.
"Just because someone has a manager or experience of the music business doesn't mean that they are any less deserving of being on a show compared to someone with an ordinary job and no experience," Contostavlos said.
"There are so many people out there who are so talented. It doesn't make it any easier for you to make it. I had a manager for years before making it and it didn't make things easier for me or make me work less hard.
"It's been a stigma before - 'Oh he's got a manager, he's in the industry, boot them out'. I think that's a load of rubbish. We've opened the door now and we've found more home-grown talent than ever."
Contostavlos recently hit back at Louis Walsh after he branded her a "chav" by asking: "When was Westlife's last number one? Because mine was a couple of weeks ago."
Watch a promo for The X Factor's ninth series, which starts tomorrow (August 18) at 8pm on ITV1, below: